I hesitate posting anything in this section since I'm presently SOOOO out of shape but I have recently read (and watched videos) quite a bit about interval training. Not only is it great for conditioning unlike run of the mill cardio your body continues to burn fat for a number of hours following a 20 minute (or so) workout. Dr Mercola has an interval training program described in free videos (Google mercola interval training). Another really efficient interval training method is called tabata. Tabata - especially in the beginning - can be accomplished in 4 minutes but don't be fooled... done properly you'll be WORN OUT in 4 minutes. Interval training in a nutshell is basically going ALL OUT for a short period of time (30 seconds using the Mercola method) then resting for 90 seconds and repeating for a total of 8 reps. Tabata is similar but the times are shorter... but for the exact workout consult the youtube videos.