Hitting the Wall in a Marathon: Why It Happens at Km 30 and How to Avoid It

Hitting the wall is that moment, usually around kilometre 30 of a marathon, when your legs stop responding and your pace collapses no matter what you do. It happens to far more runners than expect it to. This guide explains why it happens, what data from millions of marathons shows and what you can do to avoid it.

What hitting the wall means

The wall is a sharp, sustained drop in pace in the final part of the race, almost always caused by running out of energy stores. You feel it as heavy legs that will not obey, a sense of emptiness and sometimes dizziness or trouble concentrating. From there, holding your planned pace becomes impossible.

Why hitting the wall happens

Your body stores carbohydrate as glycogen in your muscles and liver, and that store is limited. The faster you run relative to your capacity, the more carbs you burn per kilometre. If the store runs dry before the finish and you have not been topping it up, you hit the wall.

A study by Benjamin Rapoport at Harvard calculated when those stores run out for each runner. His conclusion is that it depends on several things: muscle mass, how much glycogen fits in your muscles and liver and, especially, your pace relative to your aerobic capacity. With a well-chosen pace and fuelling during the race, the wall is not inevitable.

How many runners end up hitting the wall

One of the largest analyses was carried out by Barry Smyth of University College Dublin, with more than 4 million marathon results. He defined the wall as a sharp and sustained slowdown in the final part of the race, usually after mile 20, about 32 km.

According to his data, 28% of men and 17% of women hit the wall. When it happens, men slow down a little more: their pace drops by 40% compared with the first part of the race, against 37% for women. And it is more common in the years when runners are at their best, when the temptation to start fast is greatest: 36% in the three years before a personal best, against 23% in earlier years.

How to avoid hitting the wall

1. Choose a realistic pace

This matters most. Starting faster than you can sustain burns extra glycogen in the first kilometres. Set your goal with the race time predictor from a recent half, and check it during the marathon pace sections of your training.

2. Start slower than your body wants

The first 10 km should feel easy. Every minute you gain early by going faster than your pace will cost you far more at the end. An even pace or a slightly faster second half is the safest way to finish strong.

3. Eat during the race

Taking in carbs while you run delays the emptying of your stores. For efforts of two to three hours, about 60 grams per hour is recommended; start around minute 30 or 40 and keep taking something every 20 or 30 minutes. Work out your number with the race carb calculator and get your stomach used to it on your long runs, as I explain in the guide to train your gut.

4. Carb load

In the two or three days before, increase your carbs so you start with full glycogen stores. The guide to carb loading explains how much.

5. Build your endurance

Long runs of up to three hours and blocks at marathon pace teach your body to save glycogen and use more fat. They are laid out in the marathon training plan.

What to do if you hit the wall

If from km 30 you feel your pace slipping, do not fight the clock. Slow down to a pace you can hold, take a gel or sports drink at the next aid station and alternate running with a few seconds of walking if needed. Sometimes, after a few minutes of eating and easing off, your body recovers a little. What matters is reaching the finish without pushing so hard that you hurt yourself.

Conclusion

Hitting the wall happens when your glycogen stores run out, almost always because you started too fast or did not eat enough. According to more than 4 million results, it affects 28% of men and 17% of women, almost always after kilometre 32. A realistic pace, a cautious start, eating during the race, carb loading and good long runs greatly reduce your chances of hitting it.

FAQ

At what kilometre do you hit the wall in a marathon?

Usually from kilometre 30 onwards, around mile 20 (about 32 km), when glycogen stores run out.

Why does hitting the wall happen?

Because the glycogen in your muscles and liver runs out. Your pace relative to your capacity, your muscle mass, how much glycogen you have stored and whether you have eaten during the race all play a part.

How many runners hit the wall?

In an analysis of more than 4 million results, 28% of men and 17% of women.

How do you avoid hitting the wall?

With a realistic pace, a cautious start, about 60 grams of carbs per hour during the race, carb loading and long runs in your preparation.

What should I do if I am hitting the wall?

Slow down, take a gel or sports drink and alternate running with a few seconds of walking if you need to. Sometimes your body recovers a little after a few minutes.