How to avoid hitting the wall in a marathon

Photo by Semih Başaran on Pexels

Mile 20. Your legs feel like they belong to someone else, your pace has dropped by a minute per kilometre without you consciously deciding to slow down, and the finish line — which felt so reasonable on paper — now feels like a cruel joke. If this sounds familiar, or if you’re training for your first marathon and you’ve heard enough horror stories to be genuinely worried about it, you’re in the right place.

“The wall” isn’t folklore. It’s a real physiological event, and it happens to recreational and sub-elite runners far more often than to the people at the front of the race — mostly because of decisions made in the first two hours, not the last six miles. The good news is that it’s largely preventable. Not guaranteed-never-happens preventable, but preventable in the sense that most runners who hit the wall can point to one or two specific things they got wrong: they started too fast, they didn’t fuel properly, or their training didn’t prepare their body to burn fat efficiently at race pace.

This isn’t about being tougher. Mental toughness doesn’t manufacture glycogen out of nowhere. This is about understanding what’s actually happening in your muscles around mile 18–20, and making boring, unglamorous decisions in the weeks and hours before the race that stop it from happening.

What’s actually happening when you “hit the wall”

Your body stores roughly 1,800–2,000 calories of glycogen (carbohydrate) in your muscles and liver — enough to fuel around 90–120 minutes of running at marathon effort for most trained runners. After that, your body increasingly needs to rely on fat for fuel, but fat metabolism is slower and less efficient at race-pace intensities. If you’ve run out of accessible glycogen and your body hasn’t trained itself to use fat well at speed, you suddenly can’t maintain pace — not because you’re weak, but because the fuel tank is genuinely empty and the backup generator is underpowered.

This is why the wall tends to show up between mile 18 and 22, regardless of experience level. It’s a math problem as much as a willpower problem.

Train your body to burn fat, not just carbs

Here’s the part most marathon plans underemphasise: your long runs aren’t just about building endurance — they’re teaching your body to spare glycogen by using fat more efficiently at race-relevant paces. This adaptation, sometimes called “fat-max” training, happens specifically through running long, at moderate intensity, in a slightly glycogen-depleted state.

Practical application:

• Do at least 3–4 long runs of 18+ miles (29km+) in your final 8 weeks, run at a genuinely easy, conversational pace — not marathon pace.
• Occasionally run your long run on a lighter breakfast (not none) to nudge your body toward fat-burning adaptation, but don’t make this your default — chronic under-fuelling long runs increases injury and illness risk.
• Include some long runs with marathon-pace segments in the final third — e.g. 16 miles easy followed by 4 miles at goal pace — to specifically train your legs to hold form when glycogen is already dipping.

If your training has mostly consisted of shorter runs and a few 10-milers, this is genuinely the single biggest lever you can pull. A solid 16-week marathon training plan builds this in progressively, which matters — jumping straight to 20-milers without a base is how people get injured, not how people avoid the wall.

Fix your pacing — this is where most walls actually start

Ask any experienced marathoner where the wall really begins, and most will say: the first 5 miles. Adrenaline, fresh legs, a crowd — it’s easy to go out 10–15 seconds per kilometre faster than goal pace and not notice. That small overspend compounds brutally.

The fix is unglamorous: run the first 5km at 10–15 seconds slower than goal pace, every time, regardless of how good you feel. If you’re running with a pace group or GPS watch, treat the first 10km as “banking nothing, spending nothing.” Negative splitting — running the second half faster or even equal to the first — is one of the most reliable predictors of not hitting the wall. Most runners who blow up ran a positive split of 3+ minutes between the first and second half.

Fuel during the race — not just before it

A common mistake: runners carb-load properly the night before, then under-fuel during the race itself, assuming the loading will carry them. It won’t, not alone.

The general guidance from sports nutrition research is to aim for 30–60g of carbohydrate per hour during a marathon, starting from around 45–60 minutes in — not when you first feel tired, which is already too late. For context, most standard energy gels contain 20–25g of carbs, so that’s roughly one gel every 40–45 minutes, alongside water or an electrolyte drink.

Practical fuelling checklist:

• Practice your exact race-day fuelling strategy on at least 3–4 long training runs beforehand — race day is not the time to discover a gel upsets your stomach.
• Pair gels with water, not just swallow them dry — this affects both absorption and GI comfort.
• If gels aren’t your thing, options like sports drinks, chews, or even flat cola at aid stations work — the goal is consistent carbohydrate intake, not a specific product.
• Don’t skip fuelling because you “feel fine” at mile 10 — by the time you feel the need for it, your glycogen stores are already compromised.

If you’re unsure which products actually work for you, it’s worth testing a few — our honest comparison of energy gels is a reasonable starting point, and getting your carb-loading strategy right in the 2–3 days before the race matters just as much as what you eat during it.

Don’t neglect electrolytes and hydration

Dehydration and sodium depletion accelerate the perception of fatigue and can mimic or worsen glycogen-related wall symptoms — heavy legs, dizziness, and a sudden inability to maintain pace. The NHS guidance on staying hydrated during exercise is a sensible baseline, but marathon-specific hydration needs vary hugely by sweat rate, weather, and individual physiology — there’s no single number that works for everyone.

A reasonable approach for most runners: drink to thirst rather than forcing a fixed volume, and include an electrolyte source (not just water) especially if you’re a heavy sweater or running in heat above 18–20°C. Both overhydration (hyponatremia) and dehydration cause problems, so “more is better” isn’t the right mindset here.

Build in race-specific long runs, not just mileage

Hitting 20 miles in training is a milestone, but how you run that 20 miles matters more than the fact you completed it. A 20-miler run at an easy, chatty pace with no fuelling practice teaches your legs very little about surviving mile 20 of an actual marathon at goal pace.

Better structure for your final 2–3 long runs before taper:

• Run the first 70% easy, genuinely easy — the kind of pace discussed in our piece on the difference between easy and recovery runs.
• Run the final 20–30% at goal marathon pace, practising your exact fuelling and hydration plan.
• Finish these runs tired but in control — not destroyed. If you’re falling apart by the end of training long runs, that’s useful information to adjust goal pace downward, not a sign you need to push harder.

Respect the taper — this is not optional

The two to three weeks before race day exist specifically to let your muscles fully restock glycogen and recover from accumulated training fatigue. Cutting your taper short because you feel “under-trained” is one of the more common and avoidable causes of hitting the wall — you arrive at the start line already partially depleted.

Reduce volume by roughly 20–40% in the final three weeks while keeping some intensity (not just slow, easy miles) to maintain neuromuscular sharpness. Trust it. The fitness is already built; the taper is what lets you access it on the day.

The Honest Takeaway

• Run the first 5km of the race 10–15 seconds/km slower than goal pace, no matter how good you feel — this single change prevents more walls than any gel ever will.
• Start fuelling by 45–60 minutes in, aiming for 30–60g of carbs per hour, and rehearse this exact plan on at least 3 long training runs beforehand.
• Prioritise long runs that finish with marathon-pace miles, not just distance — your legs need to practice holding form while glycogen-depleted, not just logging kilometres.
• Don’t shorten your taper out of anxiety — a 20–40% volume reduction in the final 2–3 weeks is what lets your training actually show up on race day.
• If you do hit the wall despite all this, it’s not a verdict on your fitness or your worth as a runner — walk breaks, slowing down, and finishing anyway are all valid parts of a marathon, not failures of it.

Next read: Get your carb-loading right before race day