During long-duration exercise, the body generates energy primarily through aerobic metabolism, using oxygen to convert carbohydrates and fats into energy. Initially, glycogen stored in the muscles provides a swift energy source, but as activity continues, the body increasingly relies on fat stores for fuel. This process is efficient for endurance activities, allowing sustained performance over prolonged periods. Additionally, the body may deplete glycogen reserves, which can lead to fatigue, emphasizing the importance of proper nutrition and hydration before, during, and after long-lasting exercises to maintain energy levels.