Not as uncommon as you think. The pushup moves you can do from your knees you can do that. Some are moving so you can't really do that so I suggest you go down a little, not all teh way and do mini pushups. Online support can be a great tool for getting through an INSANE program like this and I’ve got an awesome group of people doing Insanity and we’d love to have you join us for support, motivation, inspiration, information and friendship. Check us out here:
Larry's INSANE Thread]