P90X is a pretty great thing, but it my view, jumping straight into it from being a couch potato is efficient, but if you go from never working out to working out 6 days a week, and dropping all bad habits cold turkey, it's not really the first week that's hardest if you're determined, but when your body starts fighting back is when you have to cope with something you've never experienced before.
I did a month of P90X a year ago. I never got back into it after that first month, though. I gave my body a break since it was summer, and it took with both hands. Now I'm gonna do the same thing again, but I'll probably only be working out four days a week instead and not have a huge calorie deficit. Slow and steady really, really wins the race. If you're super determined, or already rather fit, you can cope, but for those of us that want to be in healthy shape, but doesn't need to take it further, designing a program that will always be doable for you is what's paramount. You can do P90X, but use it the way you want. It's a great program that allows you to do everything at home, and if you dare go beyond what you're thought there, you are on a good track.