Hitting the Wall

Hitting the wall, sometimes called bonking, is a sudden, severe drop in energy during a long run, usually in the second half of a marathon, where pace collapses and effort feels disproportionately hard. It's largely preventable with the right fuelling and pacing strategy, which makes it one of the more frustrating race-day problems, since it's often avoidable in hindsight, and the runners who experience it once tend to spend the rest of their racing careers actively planning around it.

Why it happens

Your body stores a limited amount of carbohydrate as glycogen in muscles and the liver, generally enough to fuel roughly 90 minutes to two hours of continuous running for most people, though this varies by individual and training status. Muscle glycogen fuels the muscles doing the work directly, while liver glycogen helps keep blood sugar stable along the way; both draw down together during sustained effort. Once glycogen stores run critically low, your body shifts toward burning fat for fuel, which it can do almost indefinitely but nowhere near fast enough to sustain race pace, and the result is the sudden, heavy-legged crash runners describe as hitting the wall. It's not a gradual slide so much as a comparatively abrupt one, which is part of what makes it feel so unfair when it happens: everything can feel fine, and then within a kilometre or two, it very much doesn't.

Who this is most relevant for

Hitting the wall is mostly a marathon-distance concern, which is why marathon-specific fuelling advice exists as its own topic rather than being folded into general race-day nutrition. For a full marathon, it becomes a genuine risk without proper fuelling, which is why how to fuel a marathon is worth reading in full alongside this piece, particularly if this is your first time racing the distance.

Early warning signs you're heading toward the wall

• Pace drifting down without a matching drop in perceived effort, you feel like you're working just as hard for a noticeably slower pace.
• Legs feeling suddenly heavy or hollow, distinct from the normal, gradual tiredness of a long effort.
• A foggy, irritable, or unusually flat mental state, sometimes described as struggling to do basic maths on pace or time.
• An unusual craving for something sweet, or a sudden, sharp hunger appearing mid-race.
• Stride and form breaking down earlier than they normally would at a similar point in a long run.

None of these signs mean you've already hit the wall, but noticing one or two of them is a cue to fuel immediately and ease pace slightly, rather than waiting to see if they pass on their own. Catching the pattern early is far more effective than trying to recover once the wall has fully arrived.

Preventing it through fuelling

Starting to fuel early in a marathon, rather than waiting until you feel low on energy, is one of the most consistently repeated pieces of advice among experienced runners and coaches. By the time you feel the effects of low glycogen, you're already behind on replacing it, since digestion and absorption both take time, and a gel taken the moment you feel depleted won't help you for another 15 to 20 minutes, well after the damage to your pace is done. Fuelling and hydration also work together rather than separately; taking in carbohydrate without enough fluid to go with it can slow absorption and cause stomach discomfort, which is its own way of derailing a race. See carb loading explained for how pre-race fuelling fits into the picture, alongside fuelling during the run itself.

Preventing it through pacing

Starting too fast is one of the most common contributors to hitting the wall, since it burns through glycogen faster than a controlled, even effort would, and it does so precisely when you have the least ability to tell that it's happening, adrenaline and fresh legs both mask the true cost of an aggressive opening pace. A pacing strategy that holds back slightly in the first half, even if it feels conservative, tends to produce a much stronger second half than an aggressive start followed by a fade. See how to pace a negative split for the fuller version of this idea applied across an entire race.

• Start a notch slower than your goal pace for the first few kilometres
• Begin fuelling early, generally from around 45 minutes to an hour in, rather than waiting
• Practice your race pacing and fuelling plan on long training runs beforehand
• Avoid making up time aggressively if you start slightly ahead of schedule

Common mistakes beyond pacing and fuelling

• Neglecting hydration and electrolytes alongside carbohydrate, since dehydration compounds low glycogen and can bring on wall-like symptoms even with adequate fuelling.
• Never rehearsing race nutrition on long training runs, so race day becomes the first time trying a new gel, chew, or sports drink.
• Ignoring early warning signs and hoping they pass, rather than adjusting pace or fuelling the moment they appear.
• Underestimating how much a hot or humid morning increases carbohydrate and fluid demands compared to training runs done in cooler weather.
• Under-eating in the day or two before the race, arriving at the start with lower glycogen reserves than a normal training day would leave you with.

Hitting the wall versus just being tired late in a race

It's worth telling the two apart, since they call for different responses. Normal late-race fatigue is a gradual, proportional increase in how hard everything feels as distance and time add up, expected and manageable with pacing and mental strategy alone. Hitting the wall is a comparatively sudden, disproportionate collapse, not simply the race getting hard, but a distinct physiological shift where the fuel your muscles were relying on has genuinely run short. Mistaking normal fatigue for the wall can lead to over-fuelling or panicking unnecessarily; mistaking the wall for normal fatigue, and trying to push through it with willpower alone, tends to make the final kilometres considerably worse than they needed to be.

Heat, humidity and hitting the wall in India

Indian races often run warmer and more humid than typical fuelling guidance assumes, and heat changes the equation in two ways at once. It increases the body's reliance on carbohydrate for a given pace, since working in the heat is metabolically more demanding, and it increases fluid and electrolyte loss through sweat, which can bring on wall-like symptoms earlier than they'd otherwise appear even with reasonable fuelling. This is one more reason early, proactive fuelling and hydration matter more, not less, in typical Indian race conditions, and why pacing conservatively in the early kilometres matters even more when the back half of a longer race may also be warmer, since most events start early specifically to buy cooler running time before the sun climbs.

If it happens on race day anyway

Slowing down, taking on whatever carbohydrate source is available, and walking through an aid station if needed is a more effective response than trying to push through at the same pace. A gel, a sports drink, or even the fruit and biscuits sometimes offered at aid stations on longer routes can help, though the effect isn't instant, so pair it with a genuinely easier effort for the next few minutes rather than expecting an immediate turnaround. Recovering fully from hitting the wall mid-race is difficult, but easing off early usually limits how bad the last few kilometres get, and finishing at a slower, controlled pace beats a stop-start struggle that risks injury or a much longer recovery afterward.