Well, I had an athlete sim and when she exercised at work it added hours, so i'm assuming it does for the other careers too... the ones that allow you to exercise. I haven't tested it much though, but my athlete sim got the challenges with ease since she exercised at work all the time.
For marathon runner, I just had her jog around the town. That one's easy. But for cardio and strength, you almost have to do it at work because it requires a ridiculous amount of hours to complete.
[Edited for punctuation. Please be sure to use correct punctuation when posting. Thanks! -Pam]