If you're like me and you hate doing 45 minutes of regular cardio, then try something called HIIT. This stands for 'High Intensity Interval Training'. The theory behind this is what explains how sprinters look like they're in better shape than cross-country runners. HIIT training is more effective at burning fat while maintaining muscle mass.

Here's an example of a HIIT routine:

1. Jog for 5 minutes to warm up.
2. Sprint for 30 seconds.
3. Jog for 1 minute.
4. Repeat 2nd and 3rd steps 4 to 5 times if you're new to HIIT. As you get used to this, you can methodically add another 30 second sprint.
5. Jog for another 5 to 10 minutes to cool down.

This works wonders for your metabolism. It can be done with running, biking, stationary bike, treadmill, etc..