I don't know much about P90X sounds like you want to dance from the ZUmba and the Yoga Booty Ballet.
Here's my thing, I modify modify modify... and I don't berate myself for it. If it HURTS too much to do something i don't do it. I substitude toe taps, or marching in place, or knee-ups for some of the jumping low impact cardio still raises your heart rate.
Even Just making a "dance" playlist and being sure to include moves that get your heart pumping for 10 minute sessions can help. Use LOTS of arm movement, keeping your arms moving above your heart adds a lot more cardio benefit to the excercises you are doing.
The "Biggest Loser" DVD's are pretty straight forward, I don't know if you've looked at those, i have one that has options so you can start with the really easy, really low impact combination of excercises to get you started and use to excercising.
I take 3x a week aerobic classes, and started when I was heavy and out of shape, just going and making sure I kept moving evne if I couldn't do the moves helped. I have bad knees and modify all the jumpy parts to suit me sure I am not getting quite as much of a cardio bomb as everyone else but i'm still elevating my heart rate and thats good.