I'd like to add a few things to jppe's post.
The basic premise of jppe's post is correct: fewer calories in than burned results in lower body weight. What needs to be added is that your body burns sugars/fats/muscle in different proportions that vary with the intensity of the activity. Very intense workouts burn the "premium" fuels (sugars and muscle) while less intense workout can better access fat stores. The bottom line is that if you want to lose a lot of fat you will have to put in long hours at relaxed pace; the process cannot be hurried. Search for some of Dannoxyz's posts in the Training and Nutrition forum for more details.