Be sure to warm up properly before this workout! Definitely warm up your entire body and be prepared to move fast from the start.
:20 on, :20 off
:30 on, :10 off
:40 on, NO REST!
Big Flutter Kicks
Work hard during each interval! Just because that first interval is only 20 seconds, doesn't mean you should take it easy. Those 20 seconds should be spent going ALL OUT and your legs should be burning!
Give it a try and let me know how it goes!