The primary source of energy for the body during prolonged exercise is glycogen, a stored form of carbohydrate found in muscles and the liver. Glycogen serves as a readily available energy source that can sustain activity over extended periods. As exercise continues beyond 30-60 minutes, the body begins to utilize fat stores for energy as glycogen reserves deplete. Both glycogen and fat metabolism are essential for endurance activities, highlighting the importance of proper nutrition and energy management for athletes and those engaging in long-duration exercise.