Even if you can't stick with the entire P90X program on your trips it's worth having as you can do almost all the workouts in a hotel room. The exceptions are the upper body workouts that require a pull-up bar or free weights. Of course if your goal is to just stay/become healthy, you can tailor the program to what you need. The other parts of P90X deal with cardio, flexibility, core, etc which can all easily be done by pushing the bed out of the way in a hotel room.
If you just do what you can in a hotel room, supplemented with various types of pushups for upper body, then you'll be most of the way there. There is a fitness test that you should at least read before you begin P90X as many pilots are not in good enough shape to handle the moderately intense workouts right off the bat. You probably won't need to actually do the fitness test, but just reading it (and being honest with your self-eval) you'll know. I've spoken with several guys that pushed it too hard, too fast and ended up hurting themselves. One of them did a pretty good number on his knee. Just ease into it (as much as you can with P90X) and you'll be fine.
As far as food on the road, I second what the others have said. I usually pack a small lunchbag size cooler with a sandwich or two for the first day of the trip, and as much fruit as I can squeeze in there.
Eventually you'll run out of your stash and have to go hunting/gathering in the terminal or around your layover hotel. I've made a huge effot to cut out french fries (I only allow myself fries once a month, max). If you're eating at a restaurant avoid adding cheese and mayo to burgers/sandwiches - you'll hardly notice they're not there and you'll be skipping a ton of fat/calories/cholesterol. When you get a sald, have the dressing on the side and ONLY use a very little bit of dressing; just enough to choke down the lettuce. If you don't know what I'm talking about, read the back of the Ken's dressing packet for the nutrition information. If you drink the whole dressing packet, you might as well trade the salad in for a cheeseburger.
Speaking of burgers, I flew with a guy that had a rule for himself. If he had to have that cheeseburger on a layover, he would substitute brocolli or steamed carrots for the fries. This kept him from: A) eating the fries and B) forced him to eat a few veggies along with his indulgence.
I always try to order water from the FA's and when eating out. I prefer my liquid calories in beer so I try not to waste them on soda during the day
Summary: eat your fruits and veggies and stay somewhat active on layovers and you can avoid being the waddling pilot that we all see in the McDonald's lines at airports around the world.