[nethergoat]

For those already in decent shape, here's a way to maximizing the use of those 20 minutes: high-intensity interval training (HIIT - http://en.wikipedia.org/wiki/High-intensity_interval_trainin...). The basic premise is to alternate between maximum effort (sprinting) and medium effort (jogging) at set intervals. The key is that the sprints are anaerobic rather than aerobic. This type of workout induces a state of excess post-exercise oxygen consumption, thereby raising your resting metabolic rate for about 24 hours.

Hop on a treadmill (or bike, elliptical, whatever), warm up for 5 minutes, then go all-out for 30 seconds. Jog for 60 seconds, then go all-out for another 30 seconds. Repeat the cycle 6-8 times. Cool down for 5 minutes.

Best bang for your 20 minutes. Do it just three times a week; add in lifting, normal cardio, etc. as desired.