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I Ran 5K Every Day for 28 Days – Here’s What it Did to My Body, My Fitness and My Strength

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I Ran 5K Every Day for 28 Days – Here’s What it Did to My Body, My Fitness and My Strength

The challenge was simple: run 5k every day for 28 days. No rest days, no shortcuts, no doubling up to get ahead. Just 5k, every day, regardless of how I felt.

I wasn’t starting from scratch, but I wasn’t in peak running shape either. My training had been half-hearted for a while – enough to get by, but not really enough to improve. I knew 5k was doable on any given day. The question was what it would feel like doing it 28 times in a row, with a sleep-shy toddler, work and general life distractions thrown in. I also kept strength training twice a week throughout – mainly because I didn’t want my gym numbers to completely fall off a cliff.

The plan – if you can call it that – was to hold everything steady and see what gave first.

Isaac Williams

Isaac after one of many Crystal Palace Park laps

5K a Day

The first week felt like a chore more often than not. A couple of runs came straight after the post-work commute, when motivation was low and the run itself didn’t do much to improve things. One evening in the rain was particularly grim. Another came off the back of no sleep, was left until late and done purely because it had to be.

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The gym treadmill made an appearance one lunch break, which was an immediate mistake. Five kilometres indoors felt far longer than it should, even when breaking it up with intervals just to stay engaged.

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By the second week, fatigue started to build. Day nine stood out as the hardest – low energy, heavy legs and no real explanation beyond the fact that everything was adding up. It didn’t feel like I was getting fitter, just more tired.

Around the middle of the challenge, things became more manageable. The daily effort started to feel less like a battle and more like routine. By day 20, it was just part of the day.

Daily Habits

Our fitness director, Andrew Tracey, explains the rationale behind daily fitness challenges.

‘If you’re looking to make a habit really stick, to the point where it’s almost automatic, then daily adherence can remove a lot of decision fatigue and deliberation. A study in the European Journal of Social Psychology found that, on average, it takes 66 days to achieve automaticity with a habit, but that daily adherence accelerated this time greatly.

‘That being said, “daily workout challenges” tend to be found at the extremes: Facebook mums committing to 30 squats a day for a month (no shade, a great thing to do), or YouTube men documenting their weird new obsession (“You won’t believe what happened when I did this for 30 days…”). They also tend to veer from harmless, and usually fruitless, to ill-advised and self-destructive. But there’s a lot of evidence in favour of building daily habits – especially fitness ones – and the benefits these can have on the rest of your life.’

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isaac run streak

Isaac Williams
isaac run streak

Isaac Williams

Progressive Overload

I wasn’t new to running, but if you are, heed this advice from Daine McKibben Rice, director of Validus Sports Injury Clinic.

‘Running 5k every day for 28 days is less about distance and more about load tolerance. For most people, 5k daily represents a pretty significant increase in cumulative load, particularly if they’re not already running regularly. The strongest evidence shows that rapid increases in training load are the primary driver of running-related injury, rather than distance alone.

‘For beginners, the risks are higher. Around one-third of recreational runners sustain injuries annually, with overuse injuries being the top of the list: such as tendinopathy, shin splints or patellofemoral pain. This can be due to repeated loading without sufficient conditioning or recovery, as musculoskeletal tissues take more time to adapt compared with cardiovascular fitness.

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‘A gradual build-up is essential. For beginners, increasing running volume by 5% per week and progressing from two to three runs weekly towards consecutive running days is a safer approach than starting with daily 5k runs.’

mudcovered shoes on grass
Isaac Williams

It wasn’t all sunshine and rainbows…

The Verdict

By the final day, I wasn’t desperate for it to end, which felt like an achievement in itself. I finished with a 5k time trial and came within 30 seconds of the 20-minute mark – not an all-time PB, but the quickest of the year and better than expected given there hadn’t been a single rest day in four weeks.

The more interesting result was how quickly my body adjusted to running every day. Individual runs didn’t suddenly feel easy, but they stopped feeling like a big deal. It became normal to head out, get 5k done and move on.

‘At the end of the challenge, I hit a two-rep squat PB’

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The strength training was where things got confusing. I expected my numbers to dip, or at least stall, but the opposite happened. At the end of the challenge, I hit a two-rep squat PB. Which either says something about the benefits of consistent training or suggests I’d been underperforming before. Possibly both.

In body composition, not much changed. I didn’t lose muscle, but I didn’t drop any body fat either. That’s probably down to eating more to keep up with the extra running. Daily cardio on its own isn’t a guaranteed shortcut to getting leaner, especially if you’re fuelling properly. And to be fair, that wasn’t the point of the challenge anyway.

Running every day isn’t something I’d recommend in the long term and it’s not the smartest way to improve performance. But as a short, controlled block of consistency, it does exactly what you’d expect: it builds a habit, raises your baseline and proves you can handle more than you think.


Headshot of Isaac Williams

Isaac Williams is Site Editor for both Women’s Health UK and Men’s Health UK, guiding and supporting the content teams to create content across all platforms. Isaac’s love of health and fitness began at Loughborough University, where he graduated with a History and English degree in 2014. His first job was at Men’s Running magazine, where he progressed from Staff Writer to Editor. Among his highlights of those four years include completing a 24-hour track race (never again), just about finishing a multi-day ultramarathon in the Azores, and chugging his way around a ‘beer mile’. Isaac ventured into the world of freelance journalism in 2018, interviewing some of the biggest names in sport – like Anthony Joshua and Ben Stokes – and writing features for the likes of The Guardian, Red Bull, ShortList and BBC Countryfile. He was also a regular contributor to an adventure series called ‘The Man Who’: speaking to some of the world’s most extreme explorers from the wilds of Caffè Nero. In late-2019, Isaac became Editor of Men’s Fitness UK. In his five years there, Isaac was responsible for editing the monthly magazine and managing website content, ultimately helping the brand transition to a ‘digital-first’ approach. He joined Hearst UK as Multiplatform Editor in December 2024, where he manages day-to-day digital output, edits content and writes articles on all things health and fitness. When he’s not hammering at his keyboard, Isaac enjoys exercise and trying –  unsuccessfully, so far – to teach his baby son to kick a football. You can follow Isaac on Instagram @isaacw1993. 

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Why 21-15-9 Might be the Smartest Workout Format in Fitness – and How to Use it to Drive Muscle Growth

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Why 21-15-9 Might be the Smartest Workout Format in Fitness – and How to Use it to Drive Muscle Growth

CrossFit means a lot of things to a lot of people – because it’s made up of a lot of things.

Since the rise of the fitness giant, countless brands, events and training methods have sprung up around it – not claiming to be CrossFit, but looking suspiciously CrossFit-esque.

There are, however, a handful of things that are uniquely CrossFit: the ‘Girls’ benchmark workouts. The Hero WODs and, of course, its signature rep schemes.

Chief among them is ’21-15-9′.

The 21-15-9 rep scheme may just be the single most CrossFit thing in existence. But what exactly is it? Where did it come from? And why might it actually be better at building muscle in a hurry than its conditioning roots would have you believe?

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Let’s have a look.

What Is 21-15-9?

If you’ve never encountered it before, the format couldn’t be simpler. Choose two exercises (occasionally more) and perform 21 reps of each, then 15 reps of each, then nine reps of each, completing the entire workout as quickly as possible – with good form.

Probably the best-known example is ‘Fran’: 21 thrusters and pull-ups, followed by 15 of each, then nine. On paper it doesn’t look especially intimidating. In practice, it’s one of the most feared benchmark workouts in fitness.

Where Did it Come From?

Unlike many modern training methods, 21-15-9 didn’t come out of a study. It came from the gym floor.

CrossFit founder Greg Glassman has explained that the format emerged through years of coaching and experimentation in the 1990s. Rather than chasing a perfect sets-and-reps prescription, he was looking for a workout that allowed athletes to maintain a high power output from start to finish.

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The thinking is surprisingly elegant. You begin with 21 reps while fresh. By the time you reach the set of 15, your ability to produce force has already fallen. By the final nine, you’re significantly more fatigued – but the workload has dropped by almost the same amount.

Instead of grinding through increasingly miserable sets of the same length, the workout ‘meets you where you are’, reducing the work required as your capacity declines. The result is a workout that encourages you to keep moving instead of standing around trying to recover.

The numbers themselves are also remarkably practical. Forty-five total reps per movement provides plenty of training volume without turning the session into an endurance slog, while every set divides neatly into thirds if you need to break it up.

(Although I’ve got to be honest, I’m a 20-15-10-5 man myself, just for the sake of round numbers.)

Why Does it Work So Well?

Although there isn’t research showing that 21-15-9 is somehow the magic formula, there are obvious reasons why it consistently produces brutally effective workouts.

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Descending reps help maintain intensity. As fatigue accumulates, reducing the target allows movement quality, bar speed and overall work rate to stay higher than they would if you simply repeated the same number of reps over and over.

It also tends to land in a physiological sweet spot. Most 21-15-9 workouts take between three and eight minutes, depending on the movements and the athlete. That’s long enough to create a serious cardiovascular challenge while still requiring meaningful force production throughout. You’re taxing your anaerobic systems hard while relying on your aerobic system to help you recover just enough to keep going.

Finally, there’s the psychological trick. The hardest-looking part comes first. Once you’ve survived the opening 21, every remaining round appears more manageable. ‘Only 15 left.’ Then, ‘Just nine.’ In reality, you’re becoming more fatigued with every rep, but the shrinking target keeps you attacking the workout instead of pacing too conservatively.

Why it Might be Surprisingly Good for Building Muscle

Perhaps the biggest misconception about 21-15-9 is that it’s ‘just cardio with weights’.

Choose the right load and something interesting happens. Very few athletes complete every round unbroken. Instead, the workout naturally evolves into a series of short, broken sets separated by only a few seconds of rest.

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Your 21 might become 11-5-5. Your 15 becomes 8-4-3. Your final nine might stay unbroken – or become 5-4.

In effect, you’ve accidentally turned the workout into a form of rest-pause training.

Those brief pauses allow just enough recovery to squeeze out more high-quality repetitions before fatigue catches up again. By the latter stages of each mini-set, you’re repeatedly working very close to failure, recruiting the high-threshold motor units with the greatest potential for muscle growth.

It’s a similar principle to rest-pause training, myo-reps and cluster sets: all methods used to accumulate hypertrophy-friendly volume while keeping the load relatively heavy and the rest periods brutally short.

You’re basically speed-running a large number of hard, growth-stimulating reps in a very small window of time. Could this help explain why elite CrossFit athletes often carry an impressive amount of muscle despite spending relatively little time performing traditional bodybuilding splits?

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It’s certainly plausible, although the ‘elite’ part often selects for athletes with the greatest muscle-building potential.

Much of their training isn’t simply conditioning. It’s high-density resistance training performed under accumulating fatigue, with only fleeting recovery between efforts. In other words, they’re often doing something bodybuilders have deliberately programmed for decades: packing a lot of hard work into a very short period of time.

That’s not to say 21-15-9 is superior to a well-designed hypertrophy programme. If your sole goal is building muscle, there are more efficient ways to do it.

But if you’re looking for a workout that develops fitness, tests your mettle and still provides a meaningful stimulus for strength and size, it’s easy to see why this deceptively simple rep scheme has remained one of CrossFit’s defining fingerprints for more than 20 years.

Best Bodyweight 21-15-9 Workout: ‘JT’

If you’re looking for an interesting twist on the 21-15-9 format, look no further than Hero WOD ‘JT’, which concentrates the muscle-building potential of the format into a brutal upper-body workout.

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Created in honour of Petty Officer 1st Class Jeff Taylor, who was killed in Afghanistan in 2006, the workout strips away barbells altogether and relies solely on three bodyweight movements:

21-15-9 reps of:

Don’t let the lack of equipment fool you. The volume – 45 reps of each movement, 135 reps in total – combined with the descending rep scheme makes this a brutal upper-body test, hammering the shoulders, chest and triceps while demanding serious muscular endurance.

Better still, it perfectly demonstrates one of the biggest strengths of 21-15-9. As fatigue mounts and the sets naturally fragment, the workout begins to resemble one giant rest-pause set, allowing you to accumulate a huge number of hard, near-failure reps in less than 10 minutes.

If your goal is building an impressive upper body while developing serious work capacity, there are few bodyweight workouts that deliver quite so much bang for your buck, making ‘JT’ one of my personal favourites.

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fitness magazine cover featuring a muscular man with kettlebells

If there’s one thing Kori Sampson knows, it’s how to optimise your body composition for performance. To tap into his knowledge as an elite athlete and coach, we asked him to create a 4-week plan to help you move faster, recover quicker and keep pushing when the fatigue sets in – all while improving your muscle-to-fat ratio.

Ready to build muscle, burn fat and come out the other side looking, feeling and performing better? Click here to get 14 days of free access to the plan via the Men’s Health app.


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10 minutes of swimming might not sound worth it – but I tried it for 2 weeks and found the benefits of a quick dip

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10 minutes of swimming might not sound worth it – but I tried it for 2 weeks and found the benefits of a quick dip

The concept of ‘exercise snacking’ has never been more popular. Not only is it convenient and accessible, but there is solid scientific evidence that short bursts of physical activity can yield real benefits for our health. But can a swimming workout be an effective ‘exercise snack’?

A study published in the European Heart Journal found that just 15 to 20 minutes of vigorous physical activity a week (almost as low as two minutes a day) was enough to significantly lower the risk of heart disease, cancer and early death. The study defined vigorous activity as any exercise that leaves you out of breath and raises your heart rate, including swimming.

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The Fitness Secrets of Wimbledon’s Top Tennis Pros

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The Fitness Secrets of Wimbledon’s Top Tennis Pros

While many of us are far from becoming top-ranked athletes, there’s plenty to learn from the pros when it comes to optimising our health and fitness. From Janik Sinnner’s muscle-building techniques to Novak Djokovic’s devotion to longevity, dig into these tennis pros’ secrets for peak performance.

Joris Verwijst/BSR Agency//Getty Images

CARLOS ALCARAZ

Fitness Game Changer:

Sand Footwork Drills

Any pro tennis player has to play with agility, but Alcaraz can move. To do so at a high level, the 21-year-old performs lateral movement drills in the sand, teaching his feet to drive up from an unstable surface. This can help prevent ankle injuries and build strength in his calves and shin muscles.

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jannik sinner

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JANNIK SINNER

Fitness Game Changer:

Landmine Rotations

Sinner has historically lacked the physical prowess of his competitors, so the 23-year-old has gone all in on strength and mobility work. He does landmine rotational exercises such as the hollow body landmine press, which builds upper-body power.

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novak djokavic

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NOVAK DJOKOVIC

Fitness Game Changer:

Devotion to Longevity

He’s been around this long for a reason. Djokovic, 37, eliminated gluten and dairy from his diet, started practising mindfulness techniques like conscious breathing and visualisation, and even brought a hyperbaric chamber to the 2019 US Open.

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ben shelton

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BEN SHELTON

Fitness Game Changer:

Explosive Strength Moves

Known for his consistently fast serves, Shelton, 22, relies on single-leg training, using dumbbells to do lateral lunges, step-ups, and even Bulgarian split squats. He focuses on exploding upward on every rep so he’s ready to attack the ball on each serve.

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frances tiafoe

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FRANCES TIAFOE

Fitness Game Changer:

Overcoming Isometrics

Tiafoe spent last off-season doing overcoming isometrics: exercises that force the 27-year-old to hold a position against a load he can’t move. This aids in boosting power and strength and can improve joint health.


fitness magazine cover featuring a muscular man with kettlebells

If there’s one thing Kori Sampson knows, it’s how to optimise your body composition for performance. To tap into his knowledge as an elite athlete and coach, we asked him to create a 4-week plan to help you move faster, recover quicker and keep pushing when the fatigue sets in – all while improving your muscle-to-fat ratio.

Ready to build muscle, burn fat and come out the other side looking, feeling and performing better? Click here to get 14 days of free access to the plan via the Men’s Health app.

Lettermark

Andrew Gutman, NASM-CPT is a journalist with a decade of experience covering fitness and nutrition. His work has been published in Men’s Health, Men’s Journal, Muscle & Fitness, and Gear Patrol. Outside of writing, Andrew trains in Brazilian Jiu-Jitsu, helps coach his gym’s kickboxing team, and enjoys reading and cooking. 

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