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.

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. Once glycogen stores run critically low, your body shifts toward burning fat for fuel, which it can't do fast enough to sustain race pace, and the result is the sudden, heavy-legged crash runners describe as hitting the wall.

Who this is most relevant for

Hitting the wall is mostly a marathon-distance concern. For a 5K or 10K, glycogen stores are rarely the limiting factor. For a half marathon, it can happen but is less common with reasonable pacing. 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.

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. 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. 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.

• 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

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. Recovering fully from hitting the wall mid-race is difficult, but easing off early usually limits how bad the last few kilometres get.