I know that it has been done, but the mother/son exercise story is good. Divorced/Widowed mother wants to get back into dating, or just a wife that wants to recapture her husbands attention so wants to get into better shape. The son is on or trying to get on a sports team and is working hard at keeping/getting fit. They start working out as support for each other, and get closer along the way. It could continue on to rubbing each others sore muscles and an emotional connection by a more relaxed atmosphere where they feel they can talk about things that they normally wouldn't share with each other while they do their workouts. This could all lead on to a more sexual relationship.