Here's my old job as a fitness leader advice:

similar to what robocos says, burn the calories. Its pretty old school thinking that you should have a 'target' heart rate. No matter what heart rate you have, you always burn fat. The harder you work, the more fat (as well as carbs and protein) you burn. In order to be in the fat buring zone you will have to work out for 50+ minutes... We used to call it 'long, slow, distance training'. Having a 65% heart rate does have its place in training, but unless you are extremely obese, haven't exercised for decades and/or have a specific injury then I'd recommend the following:

To make it very simple you must burn more calories than what you input (eat) in order to lose weight.

Try doing weight training as well as cardio as you will burn calories and gain muscle. By gaining more lean muscle your metabolisim increases. With a higher metabolism you burn more calories. Basically working cardio only, you burn the calories when you are only working out, but when you add weight training, you continue burning calories hours after your workout.

Hope this helps