Sprint!

I'm plagiarizing my Primal Blueprint book:

Maximum effort sprints (note: this can pertain not just to running but cycling, cardio machines, and other HIIT workouts) help increase energy levels, improve athletic performance, and minimize the effects of aging by promoting the release of testosterone and human growth hormone (these are beneficial for women as well as men.) Once a week, when energy and motivation levels are high, choose a simple brief session and go all out (here hill sprints are cited as an example.) These workouts build muscle, support enhanced organ function, accelerate metabolism, and delay aging via the "use it or lose it" principle.

The occasional series of short, intense bursts can have a more profound impact on overall fitness–and *especially weight loss*–than a medium paced jog lasting several times as long. This is because of increases in metabolic rate and appetite suppression (both due to elevated body temperature in the hours after workouts), development of more calorie-burning lean muscle tissue, and improved insulin sensitivity (working muscles learn not only to burn glucose efficiently, but to absorb glucose–transported by insulin–after workouts.)

Novices can start with three or four sprints, short of full speed, with long rest periods in between efforts. You will likely experience some muscle soreness in the days after these efforts, but your body will quickly adapt to your new workout routine. This isn't about speed–it's about effort. It doesn't matter if you aren't covering ground quickly as long as you exert yourself to the point of going all -out for that brief interval.

Eliciting a stress response (imagine cheetahs chasing you) is desirable with your sprint workouts. The difference here is that brief, intense stress is exactly what your genes crave to build fitness and strength and to build metabolic function.

I'm going to go out and do my sprints in a few minutes myself.
It's a great idea not to eat anything for about an hour after your sprinting to maximize fat burning with the release of human growth hormone. When I sprint, I feel like I am dying. About an hour afterwards I feel high as a kite. It is unbelievable good for you.

Let me know how it goes