It can be hard to go to the gym and work at speeds that will get your heart rate up into the fat burning zone. That doesn't mean you are at a loss if your not trying to go at full speed. Increasing the resistance on your cardio equipment will push your body without forcing you to go at maximum speed.
Improve your cardio load by lowering speed and increasing resistance.
Extra resistance will improve your muscle mass much like a sprinter.
The load on your cardiovascular system and heart rate is heavier which means you burn more fat.
With resistance you don't need to workout as long to receive the same benefits.