I almost mentioned the Travis McGee books, but it has been so long since I read them, I can't remember whether they all have fitness sections or if it's just a couple of them.

In one of the last books, Travis had learned tai chi. John D. MacDonald has him briefly wax eloquent about the virtues of tai chi and how it will let him stay in shape into his old age, unlike the boxing and such that had been his fitness (and fighting) styles up to that point, which were a young man's game.