Use your heart rate to HIIT your goal

High Intensity Interval Training
Low Intensity Interval Training

Interval training is a great way improve your aerobic capacity and burn fat in 30 minutes.
Calculate your target heart rate zones, 70% and 85% of your estimated heart rate max (see Feb ‘11 post).
Warm yourself up with continues movement gradually increasing intensity to bring yourself into the target heart rate zone.
Push yourself for 30-40 seconds and recheck your heart rate, you should be near the upper range of the target heart rate zone.
Rest until your hear rate recovers to the lower range of the target heart rate zone.
Repeat for 20 minutes, cool down and stretch.

In the beginning of the workout the recovery time may be shorter than at the end. Recovery time will decrease as training continues, this is a sign of improved aerobic capacity.