(Strip Number 533, Original Publication Date, 5 January 1984)
Panel 1: We start off with Elly (who's wearing the same clothing she herself thought ludicrous) asking John what he thinks HE'S laughing at.
Panel 2: At least she's joining a fitness class; all he's doing is sitting home getting tubby.
Panel 3: As she leaves, John yells "Oh, no, you don't! I'm FINE!! You can't make ME feel guilty!!!"
Panel 4: Elly tells Connie "Good. He feels guilty."
Summary: Given that Rod is a lot more athletic than John is, my guess is that making JFTF into a happy slug is Lynn's way of getting back at the man for his 'militaristic' habit of regular exercise. Well, that and probably channeling Calvin's Dad and telling her that horrible, horrible hard work breeds character.