Glycogen is the stored form of glucose (sugar) in the body. Muscle glycogen stores fuel high-intensity exercise; liver glycogen maintains blood glucose levels. The total glycogen stores in a trained runner are approximately 400–500 grams (around 1,600–2,000 kcal of energy).
At easy aerobic pace (Zone 1–2), the body uses a mix of fat and glycogen. At higher intensities — threshold pace and above — glycogen becomes progressively dominant as a fuel source. At maximal intensity, glycogen is almost the exclusive fuel.
For marathon running, glycogen management is critical. At race pace, a typical marathon runner depletes their glycogen stores around 30–35 km. When glycogen is severely depleted, the body can only metabolise fat, which requires more oxygen per unit of energy and cannot sustain marathon pace. The result is the dramatic slowdown known as "hitting the wall" (or "bonking" in cycling).
This is the physiological reason why negative splitting and conservative early pacing are so strongly recommended for marathons: going out too fast accelerates glycogen depletion, guaranteeing a worse second half.