What I did to work up to Cathe's advanced stuff was to take small breaks throughout the workouts. For example, the first time I did IMAX 2, I got through a few intervals, modifying where necessary, and then walked around for a minute or two. I then did a couple more intervals followed by a break and so on. Over time, my breaks became further and further apart until now, where I don't take any breaks at all (I still modify some of the moves that my knees don't like).
Don't give up, you will get to the advanced level eventually.
Shelbygirl