Starting with no base of exercise is a killer.
I would agree with Steve, the reason P90X is so 'effective' is because most people dive into it having never committed to an exercise program before. I would argue that you could get the same weight-loss results from just jumping rope or doing any type of cardio, and dieting.
If I were you, I would determine my desired target weight and get at or near that mark doing through dieting and cardio before you start P90X. Once you have dropped some weight, doing all those body-weight exercises will be MUCH easier. You will probably be able to achieve a higher range of motion and perform the moves correctly if you don't have the extra baggage. Also consider the less-intense (and cheaper) P90, which is tailored to those of us who are not already in great shape.