I admit, it is a bit risky. But sometimes the job requirements demand the fitness skill for an elder.
What I do is put a covered 2-in-1 bathtub next to the exercise equipment of choice. Basically I train the Sim for as long as possible, stopping on the immediate point that the Uncomfortable moodlet pops up, regardless of the overall emotion of the Sim, then dunking him in the bathtub immediately. Slipping out of the energized emotion is easy to recover with a shower.
So far they hadn't died yet. It's also useful to have the morning/night reward, as with any skilling.