I'm a full-time college student, who is disabled and doesn't have very much extra time. My doctor told me a good way for me to start out is walking every day for about an hour, because it isn't very hard on the knees, hips, and back. Next semester I'm going to start weight lifting, I'll get credit for it and I can go at my own pace.
If anyone has any more ideas for me that won't take very long or be real hard on my knees, hips, and back please tell me.

I like to swim, it's very relaxing, gets your heart rate up, and burns tons of calories, but ask your doctor first before you start anything, because sometimes swimming can aggravate pre-existing conditions.

My mother in law has alot of knee/joint problems and began working out in a pool doing low impact water aerobics and she loves it. There's not alot of pressure on the joints so she is never sore after her workouts.. it could be something to look into. Hope you find something that you are able to enjoy and stick with..