I'm in week 5 of P90X, and it's pretty good for building lean muscle and shedding extra clingy pounds. With P90X you work certain muscle groups different days, with resistance training being days 1,3, and 5 and always including the Ab video at the end. Days 2,4, and 6 are various forms of cardio, like jump training, Yoga, and Kenpo. After 3 weeks of the resistance/cardio routine, you'll have a recovery week of just cardio/core strengthening. Then when you go back to resistance training the next week, the videos change a bit so that you're working different muscles on different days. The theory is 'muscle confusion', not sure if that's why it works, or if anything where you're working out 6 days a week will produce results, but it's great.
It's been too hot where I live with the humidity to run outside, and my left knee gives me problems sometimes, so for extra cardio, I have an elliptical machine, it's lower impact than treadmill running even.