P90X has a fitness test to see if you're ready for P90X: http://www.beachbody.com/text/produc...90xFitTest.pdf
Even if you're not ready, you can definitely pace yourself throughout the workout program. It's important to note that the emphasis should be on quality and not quantity, meaning you shouldn't sacrifice form for more reps. That said, the program recommends that you keep track of your progress (ie how many reps of each exercise you do) and try to do more each time you go through the exercises. Workout sheets can be downloaded here: http://www.beachbody.com/text/produc...Worksheets.pdf
There aren't levels, but they do have three possible paths that you can try. Don't remember the names, but it equates to something like normal, extreme, and lean (more cardio-focused). Can't really say that any of these paths are really easier than the others since you're just doing the same exercises in a different order and frequency. You can always create your own path, if necessary.
It is a hard program (though I believe Insanity is even harder), but if you stick to it, you should see results. I've tried it several times before and could not stick with it (makes you feel incredibly sore the next morning), but once my gym membership expires, I'll probably give it another try.