Hi,
I know what you mean about not keeping up. For me it's with weights. I do all the exercises, but with less weight than Cathe usually uses. For instance, I use 3, 5, and 8's for the PUB biceps...and I get a great workout! I try to go up, but can't finish the reps then. For pushups I just do what I can...never as many as Cathe and the gang. Cathe says in one tape that if you use weights that challenge you, you are getting a good workout. So, I just use weights that challenge me and get DOMS and feel like I got a good workout.
I have read, too, that as you get older your body needs more time to recover from weight workouts...maybe the same is true for cardio. I basically use my bike for cardio all the decent months of the year, then do Cathe cardio in the cold months. I use her for weights though all the time. You have to listen to your body to avoid injuries!
and Have Fun!
Beth