Hello hello!
Duration is not AS important as is intensity.
Sure you can spend an hour + on a treadmill at a moderate pace and burn x-amount of calories, but as soon as you step off that treddy - all that great metabolic activity simply stops; with increased exercise intensity, comes increased metabolic activity at REST.
I think it would be great if you tried some interval training, playing around with the incline and speed control.
As an example: Speed walk on an incline of 8 for 3 minutes - rest 1 minute. Repeat 2 more times.
Next interval: Jog on an incline of 5 for 1 minute - rest for 45 sec. Repeat 2 more times, etc.
Even just speed walking on the highest incline for bouts of 1-3 minutes will surely get that heart rate up!
And plus, the stronger you get - the more you can test your limits and the FASTER you'll be able to recover in between intervals.
With the right intensity and rest periods, you can great a work out in about 15-20 minutes!
The important thing is, is that you stay within YOUR limits. Only push to where YOU feel comfortable - injury prevention is key.
Be safe,
PJ
