Bobbi -
As you return to you former cardio capacity, you might want to consider spin (sorry if this is a duplicate and somebody already suggested that). It's a fantastic, completely joint friendly cardio workout and, if you find a good instructor, it will be one of the most challenging workouts you can imagine. You might have to try a few instructors before you find "the one" who kicks your butt (or doesn't kick your butt, as you prefer). Oh, and although it's cardio, it really tones the lower body.
Not everybody loves spin (I adore it), but it might fit the bill for you. One other great thing about spin is that it's truly your workout in that you control the tension and your rpms, so it's as easy or as hard as you make it. Beware, though, that unless you've been riding a bike recently, your butt and, uh, nearby regions, will be uncomfortable after the first 2-3 times, but then you'll acclimate and never feel that again.
Cathe has a spin DVD called CycleMax that other Cathletes say is great, although I haven't done it because I don't have a spin bike at home (yet!).
Best of luck as you move forward...you can't have a better companion in your fitness journey than Cathe, so I know you'll do well.