Not to worry, if I do add injuries from exercise, I can certainly make them optional.
My intention behind base fitness is that's the level of fitness a girl will maintain without your influence. So when you stop exercising with her, she will slowly revert back to that base level. Any changes to the base fitness would need to be from significant events, for example a change in mindset, such that the girl continues to maintain a different level of fitness even when you're not involved anymore.
I'd like to add some additional methods for the player to lower a girl's fitness too & will need to think that over.