This is a question to you runners, and especially to you kirkson. If you do p90x as it's spelled out in the program, you will be doing those exercises basically every day for an hour or so a day for 90 days. The results won't be the same, as I understand it, if you spread the program out over a longer time, so that you can run.
So, your choices are to double up and do two workouts a day on days when you run, spread the program out, not as it is intended to be done, and run and do p90x, or just stop running while you do p90x.
What do you do?
When I am training for a race, which typically requires longer runs several times a week, I do the P90X resistance routine one day, and instead of the cardio program, I will run. So, for example, Monday I would do Chest and Back, Tuesday I would run, Wednesday I would do Shoulders and Arms, Thursday Run. Friday would vary, depending on the length of the run I did on Thursday or planned to do Saturday. The next P90X routine would be Legs and Back, which is a great routine, but very tough to do immmediately following AND preceding long runs. So, I might take Friday off, do my long run Saturday, then Yoga on Sunday. Starting the cycle again on Monday. P90X would have you take your day off on Sunday, but since I took it on Friday, I go staight through.
If I am not training for a race, but am running for maintenance, I often DO double up. Especially if its a resistance day. So I might do a 3-4 mile run in the AM and a resistance worout in the PM. This is usually fine, but I will say that running makes the Ab Ripper especially difficult. The first four moves in that routine are tough on the thighs, especially the same day as a run.
When NOT training for a race, I pretty much stick to the P90X program as written. However, I have found that after doing only P90X only for a while, i get a little burned out on it and will mix in some other workouts just for the sake of variety. for example, instead of P90X Yoga, I'll do another Yoga video, or instead of Plyometrics or Kenpo, I'll do another cardio workout.
You are also right that the results won't be as dramatic using this method. Also, I note that if you really want the results you see on the TV spots, you'll need to both follow the program to the letter AND follow the diet. I haven't tried the diet, but I am actually considering it. I'll post here if I try it.
All that said, its a great program, I really am in the best shape of my life at age 37, and there's enough variety in the program and ways to increase intensity as you progress that I am still sore after the workouts, as Tony would say, "in a good way!"