Don't make the mistake of treating your HIIT workout like it doesn't matter. High-intensity interval training has been found to be the best way to burn fat. The trick is doing it properly. Each 20-second work period should have you moving at the highest intensity you can muster. It's only 20 seconds, but it should feel like an eternity. You need to be breathing hard.
During the 40-second rest period, do your best to get your heart rate under control and your breath back. It won't feel like a long-enough rest, but try not to rest for longer than I've prescribed. Work hard for these 20 minutes and you'll reap the fat-burning benefits!