I agree with Judo Tom on this one.
Try sprinting when you are over 220lbs. I tried it a few times at my heaviest (227lbs), and lets just say it didnt go over to good. My legs felt like they were going to snap off.
The worst things was when I started to loose some of the water from my big cycle, I was down to about 220, and decided to do just a regular low intensity jog on the treadmill. Well ... I got into an endomorphine-zone, and ended up running for about 1hr and 30mins. when I was done I noticed that my shins were pretty painfull. The next my shins were FUCKED! It was the worse case of shinsplints (I think it was something else - someone on the board told what it could have been). Either way, my shins, felt like there were cracks all through the bone. It was like that for about 2 weeks. And do you know what ... whenever I start to run for a good length of time, I can feel the pain in my shins.
I am at 212 right now, and am still deiting down before the next cycle, but I am just starting to feel comfortable running at this weight now. Sprints are actually better because it is for such a short time, and then I slow down to incline walking.
There are other good forms of cardio out there besides running, and Interval training can be applied to most of them. The best cardio routine I have tried just recently, is a mix of "air power lifts" and plyos. I have been doing them first thing in the morning, and let me tell you this is extreme cardio. Not like riding a bike, but your heart will feel like its going to explode out of your chest. For the air power lifts, I will do squats and powerclean/pushups with just a bar, or a light weight that will allow me to pump out 100reps. Its one big circuit that involoves rotating quickly between the light power expercises, skipping, plyos (box jumps, ball hammers, bounding, medicine ball drops), punching and kicking the punching bag, and sprints. Its quite the session.
Mavy