Do you know what your anaerobic threshold is? Do you care? You should -- the higher your anaerobic threshold, the longer, harder and faster you can exercise aerobically before your muscles cry "uncle."
When you exercise, your body uses two primary energy sources: fat and carbohydrates. When you exercise at an easy level (i.e., a long, slow jog), there's no rush to supply energy, so your body can take the time to metabolize fats.
During vigorous exercise (say, one to three miles of high-speed running), your body uses more carbohydrates because they are easier to "grab" and put to use for energy than fat.
