The "fat burning zone" business is very misleading. It’s true that low intensity exercise uses more fat as fuel and that moderate intensity exercise (that you can maintain for 20 minutes or more) burns both fat and glucose. But, you're better off exercising in the aerobic zone as much as you can, because exercising at this higher intensity burns more total calories. You will burn a larger percentage of fat in relation to glucose when you are working at a lower intensity, but you will also burn fewer total calories and less total fat. The relative percentage of fat burned has nothing to do with weight loss—it's the total amount calories burned that counts.