Bonk (Hitting the Wall)

Sudden energy depletion when glycogen stores run out, usually around mile 18–20 of a marathon.

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Understanding Bonk (Hitting the Wall)

Bonking (also called "hitting the wall") is the sudden, severe energy crash that happens when your body runs out of stored glycogen — the primary fuel for high-intensity exercise. It typically strikes around mile 18–22 of a marathon and feels like someone unplugged your battery: your legs become heavy, your pace plummets, and continuing feels nearly impossible.

Your body stores about 2,000 calories of glycogen in your muscles and liver — enough to fuel roughly 90–120 minutes of running at race pace. Without mid-race fueling, a marathoner running 3:30 or slower will exhaust these stores before the finish line. When glycogen runs out, your body switches entirely to fat burning, which produces energy much more slowly.

Bonking is preventable. The strategy is threefold: maximize glycogen stores with carb-loading before the race, consume 30–60 grams of carbohydrates per hour during the race (gels, sports drinks, chews), and pace conservatively to burn glycogen more slowly. Most bonking episodes are caused by under-fueling, going out too fast, or both.

Key Facts: Bonk (Hitting the Wall)

Key facts and insights about bonk (hitting the wall) that every endurance athlete should know.

Glycogen stores hold about 2,000 calorie

Glycogen stores hold about 2,000 calories — enough for 90–120 minutes of running

Bonking typically hits between miles 18–

Bonking typically hits between miles 18–22 of a marathon

The body can absorb 30–60g of carbs per

The body can absorb 30–60g of carbs per hour during exercise (up to 90g with multiple carb sources)

Proper carb-loading increases glycogen s

Proper carb-loading increases glycogen stores by 20–40%

Pro Tips: Bonk (Hitting the Wall)

Start fueling early: take your first gel at mile 4–5 of a marathon, not when you feel tired

Practice your exact fueling plan on long training runs — race day is not the time to experiment

Carry more fuel than you think you need — it's better to have extra than to run out

If you feel the early signs (foggy thinking, heavy legs), take a gel immediately and slow your pace

Frequently Asked Questions About Bonk (Hitting the Wall)

Normal fatigue is gradual and manageable — you slow a bit but can keep going. Bonking is sudden and catastrophic: extreme fatigue, dizziness, foggy thinking, emotional instability (many runners cry), and a pace drop of 2+ minutes per mile. If you suddenly can't do basic math in your head during a race, you're probably bonking.

Largely, yes. With proper carb-loading, disciplined early fueling (gel every 30–45 minutes), conservative pacing, and race experience, bonking becomes rare. But even elites can bonk if conditions change (extreme heat), they miss a fuel stop, or they go out too aggressively.