I have to say that for a while I was known to have publicly said that Insanity is not necessarily a good workout program to use as a PRIMARY training program for distance running. My rationale was that every workout with Insanity is in your anaerobic zone and that you still need some basic aerobic training and long distance runs to see major improvement. It was while I was saying this that I had some bad luck. I fell on some ice in Philadelphia (I'm pretty sure it was planted by Phillies fans) last February and since then, my right knee on the outside edge, the IT band, was swollen. I couldn't run for more than 5 miles before feeling a nasty tug in my right leg, and would then be hobbling along for about 24 hours afterward. After consulting with a sports medicine doctor, I went on 9 months running rest, but still did Insanity almost every day, with some p90x thrown in. Oddly, even though Insanity is a high impact exercise, it never affected my knee and IT band like the constant hitting of the pavement did, so I was able to complete the exercise. I was disappointed though, because I had plans to run the Hartford Half Marathon in October of 2010 and wasn't able to when I went on running rest starting in July.
I am training for a half marathon this year, but definitely don't want to give up p90x upper body workouts! I've done a little research, both from people's personal experiences and some sources online, and figured that if I run 4 times a week and do p90x upper body workouts on the other days, I should be still be OK. Training for long distance, it's very important to include an LSD (long slow distance) run once a week, and I personally believe it is important to do at least one speed workout per week (intervals/repeats on the track, or a long-ish tempo run, at about 80-85% max heart rate). I also think that there are some major benefits to doing Insanity with running, since it is explosive work on the legs and activates the fast twitch muscles. My old cross-country coach in high school always said "A strong runner is a faster runner" and made us do circuits similar to Insanity on some mornings before school (ugh...scary memories of that!). Every once in a while, I will do a week that has 5 runs, and multiple Insanities!