I'm loving this workout. There are a couple videos and a ton of information on mercola.com
If you're looking for a challenging cardio workout I recommend checking out the Peak 8. Please take special note of the warnings. Don't overdo it when you first start because it's very challenging if you are not in great shape. I barely got through 3 reps when I started. I'm actually using a treadmill and I'm starting to feel some progress after a couple weeks, doing it for 2 weeks, 3 days a week. My endurance is really increasing.
Check it out here: LINK
