Have you seen those heart rate charts that list two "zones?" They claim that there is a "Fat Burning Zone" (less than 70% 0f maximum heart rate) and a "Cardio Zone" (above 70%). They seem to imply that if your heart rate is above the fat burning zone, your body won't burn fat.
The chart is misleading. It's a myth that you burn more fat exercising at an easier pace (and at a slower heart rate) than you do at an intense pace. Just remember: you will burn fat above the "fat burning zone" but you'll also tire more quickly because you'll be working harder.
It's true that low-intensity exercise burns primarily fat. As you increase your intensity, your body starts using a smaller percentage of fat and a larger percentage of carbohydrates as fuel. (No matter what your pace, you always burn both fat and carbohydrates.) However, here's the important point:
The percentage of calories you burn from fat doesn't matter! For weight loss, the important number is how many total calories you burn!
For example, if you walk for a half-hour, you might burn about 120 calories. If you run for a half-hour, you might burn 300 calories. If you choose the slower pace, you'll have to spend more time exercising to burn those 300 calories. On the other hand, you might find exercise more enjoyable at a slower pace. So, set a goal of burning as many calories as possible during your exercise session (say, between 300-400). Be sure to exercise consistently ( a minimum of three times per week.)
Forget about labels such as "fat burning." Just get out there and work up a good sweat while having some fun! For safety, monitor your heart rate and stay within your Target training zone 60-90% 0f your maximum heart rate (to figure this, subtract your age from 220 and multiply that number by 0.6 and 0.9, respectively).