There are several ways to improve your work capacity. The following work pretty well:
- Start with longer rest times, and shorten them each time. For example 1m work, 2m rest for one session. Then 1m work, 1.5m rest the next session.
- Multiple times a day. Essentially this is a variation of the previous point where some of the rest is much longer.
- Keep the intervals the same, but try to get a bit more work done each time. You may be totally gassed and only able to get 1-2 reps of whatever you are doing, but the next time it gets easier.
The key point is to get all the work in. You can improve over time, and probably the best bet is to keep shortening the rest times until you can get through everything. If you have to take a longer break, like 5-10m between cycles of work, that's fine when you are getting started.
Now, one of the aspects of "insanity" and P90X brands of HIIT is that you attempt to improve your work capacity with the intervals. No matter what you are doing: burpees, tire flipping, sledge hammer work, snatches, etc.--as you get stronger, and your body gets used to the conditioning aspect of the HIIT, it will become easier. Your heart rate will come down to normal more quickly, and your body won't get tired as easily.