Originally Posted by Scarecrow
I would also suggest you try out different types of cardio to keep your body from acclimating too much. Run long distance one day, run sprints another. I like biking since it is low impact on the knees.
Sound advice there. It's always best to mix it up a bit. If you have access to a punching bag, I would recommend boxercise. You'll be amazed at how tired you quickly get from punching a bag for 10 mins.
Do that and, if you can, add swimming and you'll be a cardio machine, mate. If there was one exercise I had to pick from that it would be swimming. NOTHING beats swimming in terms of cardio improvements.