Hello...I am an out-of-shape 35 year old (5'4, 180 pounds) who started p90x about 10 days ago. I was pretty intimidated at first, because when I googled p90x to see people's success stories, it seemed that everyone was already looking fabulous before they even started! I would kill to have their "BEFORE" bodies! However, my boyfriend and I decided to plunge in and I have to say it's the best thing I've ever done. I won't lie, the first week was pure hell. I could barely move my arms and legs and had trouble sleeping because whenever I moved in my sleep the pain would wake me up! But now it is week two and although my muscles still get sore, it's much more manageable. The workouts are intense but Tony (the trainer) keeps reminding you to "do your best and forget the rest". I still can't do a pullup or a proper pushup but I'm still getting a great workout with adapted moves. I have also been following the meal plan...at first it was REALLY hard to eat all that protein (I certainly was never hungry on this diet!) but I added a protein shake to my day and it really helped. After week one, I can already see some changes in my body...little bit more tone in my muscles and lost 3.5 pounds. So I've proof that an out-of-shape person CAN take on p90x! My only concern would be if you have a knee/joint injury, as a lot of the excercises are high-impact.
Hope this is helpful!