Your best bet when it comes to a fitness center or gym near Monterey Heights, San Francisco is to go up to the Inner Sunset District and to try one of the gyms there. Gym 445 or Sunset Gym.
My recommendation for a fitness center or gym near Monterey Heights, San Francisco is to go up to Inner Sunset to one of the gyms there. (Gym 445 and Sunset Gym are good, I think.) Monterey Heights are all residential so your really have to go somewhere.)
You could also use the track at City College I guess or go for a hike at Mt. Davidson Park.