Hitting the Wall: What It Is and How to Avoid It

Hitting the wall — also called bonking — is the sudden, severe fatigue that strikes when your muscles and brain run low on glycogen, their preferred fuel. In marathons it typically hits around mile 20, turning a strong run into a survival shuffle. It's preventable: consistent carbohydrate fueling, even pacing, and proper training keep the wall at bay.

What causes the wall

Your body stores only enough glycogen for roughly 90 minutes to 2 hours of hard running. As those stores deplete, your body must rely more on fat, which can't fuel fast running as readily. The result is a dramatic drop in energy, heavy legs, mental fog, and sometimes dizziness — the classic wall.

How to avoid hitting the wall

1. Fuel during the race: 30–60g of carbohydrate per hour, starting early.
2. Pace evenly: a fast start burns glycogen prematurely.
3. Carb-load in the days before a marathon to top off glycogen stores.
4. Train your long runs to improve fat-burning and glycogen storage.
5. Practice race fueling in training so your gut tolerates it.

What to do if you hit it

If the wall strikes mid-race, take in fast carbohydrate immediately — a gel, chews, or sports drink — and give it 10–15 minutes to act. Slow down, break the remaining distance into small chunks, and consider brief walk breaks. You may not salvage your goal time, but you can usually keep moving to the finish.

Training your body to resist the wall

Long runs teach your body to burn fat more efficiently and to store more glycogen, both of which push the wall further away. Combined with a tested race-day fueling plan, a strong base of long runs is your best long-term insurance against bonking.