The theory is by keeping the heart rate controlled you're burning more fat that sugar. The downside to that is it takes a lot longer to burn a pound of fat. Walking or running burns about 100 calories per mile. Running a mile takes 8-10 minutes, depending on condition. Walking a mile takes up to 20 minutes. Using the figure of 3500 calories per pound of fat, you'd have to run for 6 hours or walk for 12 hours.

The opposite extreme burns more fat. By getting your heart rate to 90% of maximum for short peroids you stimulate Growth Hormone. This causes accelerated fat-burning. Only problem there is if weight is extreme, you're likely to damage knees or hips before long.

A middle ground might be an exercise that works more muscles per day: Rowing works the heart, legs, arms and back. Martial arts work the entire body, if done right. Swimming as well works much of the body and is not so stressful to joints.