Marathon fuelling: how to not hit the wall

Why do people hit the wall at mile 20, and how do you stop it? The wall is not your body running out of fitness. It's your body running out of stored carbohydrate. Get the fuelling right and most runners can sail past the point where the under-fuelled crew falls apart. Get it wrong and all the training in the world won't save the last 10k.

Marathon fuelling has three parts: the days before, race morning, and on the move. Let me take each, and be straight about the part runners get wrong most.

What the wall actually is

Your muscles and liver store roughly 90 to 120 minutes of glycogen at marathon intensity. For almost everyone, that runs out well before the finish, typically around mile 18 to 20 for under-fuelled runners, which is exactly where you see people walking and weaving. Once glycogen's gone, your pace drops off a cliff because fat alone can't fuel marathon-effort running. The whole job of marathon fuelling is to start with full tanks and keep topping up so you never reach empty.

The days before: load up

For an effort this long, carb loading earns its place. In the 36-48 hours before, lift your carbohydrate intake towards 10-12g per kg of body weight per day while your training tapers right down. The taper is half the trick: training less lets your muscles store the extra fuel rather than burn it.

Just as important, drop the fibre in the final day or two. Switch to white rice, white pasta, bagels, bananas and honey, and ease off salads, beans and wholegrains. A gut loaded with roughage is the fast route to a desperate toilet stop at mile 6.

Race morning

Eat a familiar, high-carb, low-fibre, low-fat breakfast two to four hours before the gun. Porridge with honey, a couple of bagels, toast with jam, a banana. Aim for something like 1-4g of carbs per kg depending on how long you've got to digest it. Sip fluid steadily. And to repeat the golden rule: nothing new on race day. Whatever you eat, you've eaten before a long run in training and know it sits fine.

On the move: the numbers

During the race, aim for 60-90g of carbohydrate per hour. The evidence is consistent that runners fuelling at the higher end have better odds of hitting their goal times, including the sub-three crowd. To get past about 60g an hour you need a dual-source product, glucose plus fructose, because that's what lets your gut absorb more than a single sugar allows.

Here's the catch that's specific to running, and it's the big one: your gut tolerates less on foot than on the bike. The pounding and the jostled, upright gut of running make high carb doses harder to handle than the same dose on a turbo. A cyclist might happily take 90g an hour; plenty of strong marathoners cap out nearer 60-75g. So don't assume a cycling number transfers, and don't blame the gels if you never trained your gut to handle them.

TimeActionFirst 30-45 minStart fuelling early, don't wait for emptyEvery ~20-30 minA gel or equivalent carbs, with waterThroughoutSip fluid to your sweat rate

That early start matters. If you wait until you feel the wall, you're already too late, because it takes time to digest and the damage is done.

Hydration

Aim for roughly 400-800ml of fluid an hour, adjusted for your sweat rate, the weather and your pace. Drink early and consistently rather than waiting until you're thirsty, which is a lagging signal. Include sodium, especially in heat or if you're a salty, heavy sweater, because plain water alone over several hours can dilute your blood sodium and cause its own problems. Don't over-drink either; bloated and sloshing is no fun for 26 miles.

Train it before you trust it

Everything above only works if you've practised it. Your gut is trainable: start at 30-40g an hour on long runs and build by 10-15g every couple of weeks until you can hold your race dose comfortably at race pace. Practise on the long runs that mimic race day, not just easy plods. The runners who blow up on nutrition are almost always the ones who never rehearsed it.

FAQ

How many gels do I need? For a typical marathon at 60-90g an hour, most runners use somewhere around five to eight gels or equivalent, started early and spaced out. Work it out from your finish time and your tested hourly dose, and carry a spare.

Can't I just rely on carb loading and skip in-race fuel? No. Loading buys you maybe the first 90 minutes to two hours. Beyond that, in-race fuelling is what keeps you off the wall, for anyone running longer than that.

My stomach hates gels. What now? Train your gut first; most gel trouble is dose and lack of water, not the gel itself. If it still rebels, mix in chews, a carb drink, or whatever real-food options you've tested. The delivery matters less than hitting the number.

In MOVE we set your hourly carb target by sport and session, and running's lower gut tolerance is built into that, so the marathon number you practise with is realistic for running, not borrowed from the bike. Load before, top up race morning, fuel early and often, and the wall stops being inevitable.