One good thing about exercise is that you can build up to it. I tried the Push DVDs myself and although I liked some aspects of them, I didn't continue on with it.
What I'd do is do what you can each session. If you get too tired and can't do something, how about walking in place to give yourself a rest then try again? Don't worry too much if you can't finish the DVD (strength or cardio), if you keep at it, you will improve.
You can't out-exercise poor eating habits.