I've been working for about the last six weeks with Callanetics. Whether or not it's for you, I'll agree with the idea that's already been expressed here. Find something you can manage, and do it in small doses every other day for a while. After you get used to it, do it more frequently, then gradually add more to your routine.
That's basically what's worked for me, after YEARS of not being able to figure out how to find a routine and stick with it. So far, so good.
Rest days are important, but it's crucial to have a workout to go to the next day that's doable.
For me, the PM Callanetics in the AM/Pm dvd is the one thing I go back to when I need a workout but don't want to overdo.
Another thing you might consider is the quality of instruction. For me, it's apparently crucial that the instructor provide reliable cuing so I can do the workout without watching the screen. Workouts that don't do that just frustrate me.