To further clarify, most of my running is distances of 12km or less (you might even call them sprints). And yes, when I train aerobically, I use the HIIT method (frequent intervals of high intensity training using up all my glycogen during the race).

When I go for really long distances, like 100km bike rides; I keep my heart-rate really low (around 120-130) and use the slow-burn method (keeping my body in fat-burning mode for as long as I can). I would venture a guess that this is somewhat similar to the training method you might use when you train for your really long distance events (like marathons).

Both methods address a different kind of training.

J.