I feel that it's doing any exercise that burns the most calories, as opposed the not doing any exercise, like a LOT of people. If you like intervals, do intervals. If you like long, slow distances, then do that. Just DO something. IF you only have a short time to exercise, then intervals is the best in calories/minute. But the exercise that you like, and will continue to do on a regular basis, is better than the one that burns more calories but you hate and will quit.