The yoga sounds good if you can find classes that you can afford.

I think it would be good to find an activity that's new for both of you. Unless the person who isn't expert at it is extremely interested in the activity, trying to join in an activity with your partner that they are really good at and in shape for is usually painful for both of you.

I recently bought a bicycle from a used bike shop. They even have a lay-a-way plan. It was very affordable.

Do you feel like there's more going on here than your boyfriend just wanting to share an activity? I'm just asking because of what you said about him snapping at you about your knee hurting when you were running.