HIIT [< All you need to know. High Intesnity Interval Training.] This is the best cardio you will ever do, the shortest, the hardest, the most grueling, and will transform you. It will make ur metabolism faster for up to 24 hours, it builds cardiovascular, it builds lower body, it burn's up to 9x more FAT, than a normal 30 - 60 minute cardio session. This is what you basically do:
You start out a normal 2 minute warm up, running normal-low intesnity. (If you choose to run, this can be done: swimming, rowing, biking, etc - I run.) Or you can just start out from the get-go, 30-45 second low intesnity - medium intesnity jog. Than you SPRINT as fast as you can 30-45 second's, than jog. And you continue this for 10-15 minute's, than just jog normally to "cool down", for 1-2 minute's, and your done. I guarntee if you do it correctly, by actually sprinting as hard as you can, at the end, you will be exhausted.
Ask someone here who is into working out/nutrtion: Chrit, RXL, Emerge, etc etc. They will tell you the wonder's of this,
this is great for boxer's as well, or aspiring boxers. Because it's like the ring, 1 round it can be slow, next high intesnity.
More info:
http://www.teenbodybuilding.com/justin6.htm