Longer and faster.

As I understand it, your body begins to burn fat after the glycogen level drops to a certain level. It takes a awhile for the body to "learn" this trick, so beware of the bonk when you start training. If the goal is more about losing weight that being competitive, the longer you can get your body to burn calories...then fat, the more weight you will lose. There is probobly no perfect ratio of speed/distance, but the longer you go at a "fast" pace, the more weight you will lose over a given period.

That said, It's like many people have said; good diet trumps all.