Hahaha.
I have never done p90x, but I am also very lean and work outside. I don't think it is at all necessary to lose weight though. Like you said, diet will be huge. Ellipticals are about worthless as far as I'm concerned. You just don't do enough. I have read that walking up stairs is more effective than running on flat ground. I would keep that in mind. Honestly, I would go to the gym as much as you can. Getting motivated to work out at home when the couch, bed, tv and computer are there will be difficult. Making yourself drive to the gym will be much more effective because you will want to get as much in as you can while you are there..