There are two parts of calorie burning with exercise: those burned during exercise and those burned after exercise. You burn close to the same number of calories swimming one mile fast or slow. However, the calories burned after exercise respond more to how intensively you swam, rather than how long. The calories you burn after exercise are mostly fat, because your body is attempting to replace sugars used during exercise. To do this, it turns to fat calories to restock energy stores. Take this for what it's worth. The most important thing is that you are consistent with your exercise program.