Actually, I never did the Boot Camp workout strictly as produced (i.e. cardio lower body upper body core, then onto the next cycle); my heart rate dropped too much, plus I didn't like going to the floor as often as that workout did. I started with the Everything But Core premix and learned, again, that I didn't like the HR drop during the upper body segments, then omitted that from my ongoing "program" such as it is.
What I use in Boot Camp DVD primarily is the cardio-only premix, AND the cardio / lower-body only premix, which is deliciously fiendish when combined with another leg-circuit-style routine (as in the Body Max power circuit segment or the CTX Power Circuit circuit segment).
I think sticking in a couple of intervals in at the end of your main cardio routine, and then working your way up to more and more intervals, is a great way to incrementally build up your cardio endurance. You could start with a couple of the easier intervals, OR kick over the traces and do the hardest ones just to get acclimated. Either way, you'll build up capacity far more quickly than you think if you're consistent.
Did Rocky pick up on the fact that you just filled a CrackerJack box with sand and covered it in tinfoil to create his high step?
A-Jock