Choose the hobby for you! You already have the gear, and you have been interested for a long time. He should know that. If he doesn't, he isn't paying attention. No need to even tell him you are doing it, really, unless he asks. Don't hide it, but you don't have to advertise, either.
If the C is content to see you alone, and you like her, I don't see why not. It might eliminate the possibility of seeing her together though, in the future, so think about that before you decide!
