Do you even lift, bro?

It depends mostly on story and what I need for individuals as to exactly how things go, but most of my played characters do pursue fitness activities. If a sim absolutely must be fit to stay in character, I usually just cheat a flag to freeze their physique because my fun doesn't come from making them work out. I'm too busy with other parts of their story (and many other bits of story involving many other sims) to do it as a game-mechanic thing for every sim where fitness matters. Of course, in cases where the sim's life journey is the primary basis of the story, they have to work out the usual way (or not) and be either fat or fit or somewhere between, however it happens. I do tend to encourage many of them (sometimes even certain unplayed townies) by giving them the Gym Rat trait either by earning it or by cheat.

Edit: Also, I also flag sims for freeze physique when I don't want them to get too buff due to story reasons.