How to Pace a Marathon (Without Blowing Up at Mile 20)

Pacing a marathon is one of the hardest things you'll do as a runner. Not the running itself — the restraint.

You've tapered for three weeks. All that marathon fatigue has been stripped from your legs. You're standing on the start line feeling fresher than you have in months, surrounded by thousands of other runners, soaking up the atmosphere, the adrenaline, the energy. Everything has built up to this one moment.

And you have to go off slowly.

That is genuinely difficult. The crowd surges, the music pumps, and every fibre of your being wants to just go for it. But you can't. And here's the cruel part: it takes your body a while to realise how hard you're working. Marathon pace sits at an effort level where you might not feel the warning signs for miles. By the time you realise you've gone out too fast, you've already done the damage.

All of this combines to make the perfect storm of difficulty on marathon morning. And whilst other distances have pacing challenges too, the marathon is unique in how dramatically things can go wrong when you get it wrong. Your glycogen stores — your body's preferred fuel source — run out somewhere around 18–20 miles. If you've gone out too fast, that wall hits harder and earlier. When the wheels come off, they can quite spectacularly come off.

Finding Your Marathon Pace

There are two key pieces to the puzzle when it comes to working out what pace you should be running on race day.

Evidence from a Recent Race

A recent half marathon time is one of the best predictors of what you're capable of over the full distance. We stress the word "capable" here, because these predictors are based on averages across all runners, and you are an individual. Some runners find their marathon pace sits quite close to their half marathon pace, while others have a significant gap between the two. This isn't about being better or worse — it's just that we're all unique, and our bodies respond differently to different effort levels and intensities.

So race evidence is useful, but it's one part of the picture.

Marathon Pace Practice

The second part is actually practising the pace you want to run on race day during your training sessions. We always subscribe to effort level over pace at Full Potential, but every good rule has an exception. For marathon pace work, we'd be practising the specific pace you plan to run — or at least a window of five to ten seconds per mile either side of your goal.

We do this so we can see how your body responds to that pace. Is it realistic, or does it need adjusting? You don't need to do the whole marathon at this effort, but we'd be looking at somewhere between 75 and 90 minutes of marathon pace work woven into a longer run. That might look like a two-hour run with the middle 90 minutes at marathon pace, or a three-hour long run where you start easy and finish the second half at goal pace.

This gives you a really good idea of what race day will feel like — and whether your goal is realistic or whether you need to have a rethink.

Effort Level Over Pace

At Full Potential, we always come back to effort level as the most important guide. A pace number on your watch doesn't account for heat, wind, hills, or how you're feeling on the day. Your body does. If you use a heart rate monitor, it can be a really useful tool for keeping yourself honest in those early miles — if your heart rate is climbing above where it should be, you're working too hard regardless of what the clock says. And if you don't use heart rate, perceived exertion works just as well. We call it the Talk Test — at marathon effort, you should be able to speak in short sentences in the first half. If you can barely get a word out, you're too hot. The splits from a pace calculator give you a target to aim for, but effort is what keeps you in the race when the numbers stop telling the full story.

Pacing Strategies for Race Day

The Even Split

A really nice way of running the marathon is to keep each mile at roughly the same pace throughout the 26.2 miles. If you run it this way, you'll find that the effort level gradually rises as the race goes on, but you're able to execute each mile consistently.

No two miles are exactly the same, of course. Terrain changes, gel stops, water stations — these will all create small fluctuations. But if you've got a window of around ten seconds per mile either side of your goal pace, that gives you enough room to nibble a bit off some of the quicker miles while having a hard stop to prevent going too fast, and a backstop in case you start slipping.

Running an even split is one of the most satisfying ways to race a marathon.

The Negative Split

This means the second half of your race is quicker than the first. It's the hardest strategy to execute, but it feels the best.

You're actively holding yourself back in the first half and then picking up the pace over the second. Both of those things are psychologically tough. In the early miles, it feels like you're losing time. And there's the nagging knowledge that you're going to have to speed up later when your legs are tired.

But here's the payoff: when you're running strong at mile 20 while the people around you are slowing down, that feeling is pure gold dust. Overtaking other runners in the final miles gives you energy you didn't know you had. It is phenomenal.

You can approach a negative split in a couple of ways — either a hard pickup from halfway, or a more gradual acceleration over the course of the race. I've seen some truly outstanding performances from runners who've committed to this approach.

The Slight Positive Split

Here's something interesting — most world records are actually run with the second half being slightly slower than the first. That goes against the grain of what we've just said, but there's a logic to it. There's a natural awareness that running for this long is hard, and there will be some fatigue towards the end regardless of how fit you are.

For a runner chasing a four-hour goal, for example, I wouldn't be upset if they ran the first half or the first three quarters at around 8:55 per mile, knowing they had a little bit of wiggle room to slow slightly when the fatigue kicks in after three hours of running.

However — and this is important — this is not the same as "banking time." The idea that you should go out fast to build a buffer for later is one of the most common mistakes in marathon running. It just encourages you to go too quickly when you're feeling fresh, and the wheels will fall off later.

You will not get any interest from putting time in the bank. In fact, it will cost you dearly. Running five minutes too fast in the early miles can lead to a massive crash later on, and the margins are incredibly fine. Being a couple of minutes ahead, you might get away with. Five or six minutes faster than you needed to be? That's going to be a mess.

The Start Line Trap

One more thing worth mentioning. When that gun goes off, the crowd moves and the race might splutter for a bit. The pack oscillates, things narrow, people slow down. If you're not careful, your response is to start weaving — ducking around people, squeezing past, burning way more energy than you need to.

Unfortunately, blokes, this one is mainly aimed at you. Women tend to be much better at pacing themselves in the early miles. I know that's a gendered stereotype, but it's one that's borne out by the data — research published in Medicine & Science in Sports & Exercise found that men are significantly more likely to slow in the second half of a marathon than women. The good news is that knowing this means you don't have to fall into it. Stay patient. Trust the process. The race is 26.2 miles long, not 2.

What If It's Not Your Day?

However you plan to race, I'd always suggest having a couple of goals in mind. What makes the marathon unique — and really challenging — is that most people can only do one or two a year. Sometimes you get terrible conditions, or you end up getting ill in the final weeks, and suddenly your A goal isn't realistic.

If you get to 10K and you're struggling to stay on pace, have a B goal ready. Have something different to work towards. Reset, have a chat with yourself, and commit to the new target.

And although I don't love the term "C goal" because it suggests it's somehow lesser — completing a marathon is an outrageously incredible achievement in and of itself. Getting to that finish line is beyond a worthy goal. What I've found is that runners mentally check out when they fall behind their target, and having another goal to switch to can keep you focused and keep you moving forward.

The Edge Case: Should You Go For It?

We know that human beings love round numbers. Only two seconds separate a 3:59:59 marathon from a 4:00:01, but it feels like a world of difference. A time starting with a three feels very different from a time starting with a four.

Now, if your predicted time is around 3:32 and you want to go for 3:30, that's a decision only you can make. It might pay off. But what can also happen is you go out for 3:30, overcook it slightly, and end up running 3:45 instead of the 3:32 you were capable of.

If you're comfortable with that risk — if you're going in thinking "I'm going for this, and if the wheels come off, so be it" — then I think that's a brave and fun way to race. Sometimes it works, sometimes it doesn't. But you need to understand that you're rolling the dice, and be at peace with that before the start.

Let the App Do the Maths

We built a pace calculator into the Full Potential app because we know pacing is where marathons are won and lost. Plug in a 4:00 target, for example, and it'll give you 9:09/mile splits with gel reminders at miles 6, 12, and 18. You can see exactly what your race day looks like before you've even laced up.

There's a negative split option too, so you can plan that gradual pickup we talked about earlier. And if you've got a recent half marathon time, the built-in race predictor will help you work out a realistic marathon goal — so you're not just guessing at a target.

It's free to download, so give it a test run and see what you think.

Planning your race day strategy? Check out our race day tips and race day checklist posts. And if you're using a run-walk approach, our run-walk guide covers how to pace that effectively too.

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