Spread it!
Without question, the P90X Legs and Back DVD limits my progress more than any other workout. My legs absolutely burn throughout this entire exercise. I cannot wait for it to be over. When it is, though, you get a rest day the next day.
You do a lot of squats, lunges, tip-toe exercises, and the worst is the chair! The back is not too bad, mainly just some pull-ups. The legs part just kills you! Well, at least it does for me. Obviously, my legs are a weak point in my fitness.
To get your own copy of P90X: http://bit.ly/P90XTruth