Final answer:
For low-intensity exercise, fatty acids are the primary fuel source, while high-intensity exercise relies on glucose. Walking burns more fat than running for the same distance due to lower intensity.
Step-by-step explanation:
Fuel usage during exercise relies on various factors, including exercise intensity and duration. During low-intensity exercise, the body primarily uses fatty acids as the fuel source due to aerobic metabolism's efficiency in producing ATP. However, during high-intensity exercise, the body needs more ATP quickly, and thus it relies heavily on anaerobic metabolism, which uses glucose as the primary energy source, derived either from blood glucose or muscle glycogen. When comparing walking a mile to running a mile, walking generally burns more fat because it's a lower intensity exercise which allows the body to predominantly utilize fat stores for energy.