Why cardio exercise has diminishing returns after a few weeks

J.D. Johannes explains why it's so hard to lose weight by doing only distance running:

How can something so logical as burning more calories through cardiovascular exercise not result in sustained fat loss? The answer is in your body’s ability to adapt to exercise and the complex functions of the hormone cortisol.

To lose weight and firm up muscle tone, one apparently needs to mix in interval/sprint training:

you alternate between periods of high intensity and low intensity. . .
The primary advantage of [this], especially short sprints outdoors, is the recovery/repair signal sent to the muscles. A sprint is almost like a weight lifting set--a short burst of maximal effort involving the fast twitch muscle fibers--followed by a period of rest.

This reasoning matches my experience very well, and agrees nicely with a book I recommend highly, Run Less, Run Faster.

via Glenn