Here's a study that blew me away when I first read about it:

Group 1: 20 week endurance training program, biking for up to 45 minutes of moderate-intensity cycling

Group 2: 15 weeks, high intensity program of 15 bouts of 15-30 second sprint intervals on an exercise bike, separated by short recovery intervals 

Make a prediction: which group lost more fat? How much more?

Read on for the answer…

Group 2 lost 9 times more fat than group 1, even though they exercised for much less time.

If you like the treadmill, then run on it. Do a 30 second sprint, then slow to a walk. It's a bit of a pain on most treadmills, because it's a lot of button pushing and the tread changes speeds slowly.

If you're not committed to the treadmill, you can get an incredible workout in 12 minutes as follows (I did one this morning as part of a DVD I'm making, and it was a killer):

4 minute warmup - jogging in place, bending, twisting, arm circles, dancing, whatever. Slow and full body motions
30 seconds: run in place. High knees, pump arms, do as many as you can in 30 seconds
30 seconds: walk or jog in place
30 seconds: either squats or squat jumps, as many as you can
30 seconds: rest

Repeat 3 more times.

Total (including warmup): 12 minutes

Of course, check with your doctor, etc. etc. Make sure you're doing things with correct posture and alignment (I wish you could buy the video now, but you can't :). For squats, make sure your knees don't wobble to the sides, but keep them pointing in the same direction as the big toe. Also, keep your back and neck straight.