Body & HealthFact
Your tank of fast energy lasts about 30 kilometres.
Your body stores carbohydrate as glycogen: about 400 grams in the muscles and roughly 100 grams in the liver, together around 2,000 kilocalories. When running, the stores are used up after roughly 30 kilometres. The body then has to switch to fat, which releases energy far more slowly. Marathon runners call it hitting the wall.
Useful: Each gram of glycogen binds about 3 grams of water, hence the rapid weight drop when cutting carbs.
Source: en.wikipedia.org