Greg,
I would not worry about going over your target max heart rate especially with Insanity. Because it's designed to push you as hard as you can go for short periods of time. Then during the down time it's good to check your rate to make sure it comes back down into the normal fitness range. If you want to stay below your target then you should stick to a stationary bike, but if you want to push your limits and see what your made of, and burn a crazy amount of calories, then should go all in. They don't call it Insanity for nothing.

Coach Greg