Higher mileage is effective for losing body fat. In fact, the fat-burning zone, the effort at which you can burn the highest percentage of fat as fuel -- is higher at moderate levels of exercise. At high-intensity exercise your body will switch to burning carbohydrates. Also, your body can only maintain high intensity efforts for short periods of time. To lower your body fat, training at moderate intensities at higher mileages can work.
The human body actually burns more fat at submaximal exercise intensities. At high exercise intensities the body turns to burning glycogen. A study published in 2001 in the "Journal of Exercise Physiology" showed that fat burning declined as cyclists pedaled at higher intensities. Up to 65 percent of your energy needs will come from burning fat at moderate exercise levels. However, at high intensity your body prefers to burn glucose.
The number of calories you burn per minute is higher during high-intensity exercise, even though the percentage of fat used is lower. However, people can only exercise at a very high intensity for short periods of time. In contrast, people can maintain a moderate level of effort for a very long period of time. So, if you go for a high-mileage run, cycle or walk you can actually stay in the fat-burning zone for long periods of time and burn more total body fat than you could with a short, intense workout.
Training at higher mileage can actually increase the number and size of mitochondria in your muscles, so you become even more efficient at burning fat. Your body needs mitochondria to burn fat and the number of mitochondria in your body and their size isn't fixed. A 90-minute run, for example, provides a better stimulus to build and grow mitochondria than a 60-minute run. But beware -- there is a diminishing return at some point. A three-hour run provides a better stimulus to build mitochondria to your body than a 60-minute run, but it takes longer to recover before you can have your next workout.
Start exercising on an empty stomach to burn the maximum amount of fat. It's common practice for athletes to snack before exercising. However, eating raises your blood sugar, which in turn stimulates the release of insulin from the pancreas. Insulin blocks the removal of fat from fat deposits. So, to burn the maximum amount of fat during exercise, don't eat for at least two hours before training. If you need to have a snack during exercise, the body is less likely to release as much insulin.
While you can lower your body fat at high mileages, there is evidence to suggest that high-intensity exercise provides the same results in less time. A study published in the "Journal of Obesity" shows that in overweight men, sprinting for 20 minutes three times per week was sufficient to lose 1.5 kg in 12 weeks. According to the authors, men would need to jog for seven hours a week to see the same results they get in an hour of sprinting. The study's control group of participants did no exercise.