Insanity

Honestly I give big kudos to you for being willing You have a couple options:
1) modify moves. If you find you are getting too much impact from the jumps and such, modify so you don't get hurt
2) Take the breaks that Shaun says to take when you need them. If you are working up the sweat, you are STILL accomplishing something and should feel proud!! Take them often if you need to - there is no pressure to keep up with the "machines" he has doing this on the dvd with him
3) Work on your fitness level until you are able to do more and then kick it out again.

I am doing Insanity now and it is tough. However, that is why I wanted it. It's too hot here in AZ for my running right now, and it seemed like the perfect opportunity to try. I am on the floor a bunch and I am soaked head to toe at the end of a workout... but you also need to make the decision that is right for you. If you need to build up first in order to feel like you are accomplishing something, then do it. If you can feel you are accomplishing something by following option 1 or 2, I say do it!