I am older than you and do most of Cathe's workouts. When Step N Motion 1 first came out, I could only do part of it and found those static lunges off the step to be very tiring. Even now, I seldom do all of Maximum Intensity Cardio and prefer the step portion of it. For me the most challenging workouts are Power Max and CTX Leaner Legs. I have all of Cathe's strength workouts and most of her cardio workouts. I concur that the CTX Kickboxing one and Cardio Kicks would be good with modifications. In addition, consider the CTX series, since the cardio portion is short, the strength portions are about 10 minutes, and there are varied abs sections including planks. With that series, you can do as much as you can handle and gradually progress. The workouts I do the most are Pure Strength, Maximum Intensity Strength, the Pyramids, Body Max, CTX and Ab Hits. Recently, I have lowered the step height to 6" and have learned the hard way to listen to my body and not go crazy overdoing it. I did that last spring when the Intensity Series came out, because I enjoyed the workouts so much. (I am modifying high impact moves a lot more lately.)
I have always exercised more for health benefits than for appearance, but even in my fifties, I gained great improvements in my physical appearance. For example, I have better shoulders and posture now, thanks to strength training.
Good luck with your workouts. What a great forum at which people of all ages share their experiences with exercise videos and DVDs.
Bea