It sounds you are assuming that burning 800 calories a day in exercise will in itself lead to loss of excess body fat. This is not necessarily true. It is necessary to strictly control your calorie intake in order for increased activity to lead to fat loss.
It is much more effective to lose weight through dietary manipulation than exercise.
What sort of exercise are you planning to do? Endless miles on the road, a treadmill, an elliptical, or a bike? Moderate exercise burns 200-400 calories per hour. That's a lot of hours grinding away at it. Most people find weight lifting more effective for body recompositioning.