Here's a plan with just two 20s sprints with 3mins in between:

http://www.highoctaneride.com/frequ...sked-questions/
(click "The exact protocol")

They make out you need a special bike but I just set mine to a fairly high resistance (such that I can only just finish the 20s) and pedal very slowly in between the sprints. My bike also has arm handles attached to the peddles so I work more of the body.