You can accomplish your cardio workout goal in 20 minutes per session by increasing and varying the intensity. It is called High Intensity Interval Training (HIIT). It is the best way to burn fat in the shortest amount of time. This training method is also used to increase speed and VO2 Max.
The danger of doing cardio exercise at the same pace (like 65% of your max heart rate) and same time period is that your body adapts and conserves energy. By doing HIIT, your body doesn't have a chance to adapt, and you burn more calories and fat in a shorter period of time. HIIT also raises your heart rate faster and keeps your metabolism elevated longer after your workout. So how does HIIT work? Here is an example:
1) Use any cardio activity like the treadmill, bike, elliptical, etc.
2) Warmup on the treadmill for 5 minutes. Exercise at 75% of your maximum heart rate for 1-2 minutes. Then walk or jog for 30 seconds to 1 minute to recover. Complete this cycle for 20 minutes increasing the intensity each time. Intensity can also be increased by using an incline on the treadmill or increasing the resistance on the bike. You should vary your intervals each workout to keep your body guessing. Cool down for 5 minutes and do static stretches after your workout.
You should get medical clearance from your doctor before starting this type of exercise. If you are a beginner, you can still do interval training. Beginners can do intervals at lower intensities. All it takes is 20 minutes to exercise!
In part 6 of this series, I will detail a better way to strengthen and tone your abdominal muscles.