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 carbohydrate. When you exercise at an easy level (i.e., a long, slow run), there's no rush to supply energy, so your body can take the time to metabolize fats. However, during vigorous exercise (say, one to three miles of high-speed running), your body uses more carbohydrates because they are easier to "grab…