There’s more than just sweat on the tiny white Detroit Tigers rally towel I used to wipe my sparkling forehead throughout last Thursday’s Jazzercise workout. Maybe some tears, too. It was my first day back in six months, after stopping to focus on preparation for May 31 heart valve surgery. I was actually nervous before class. Somehow worried that I’d forgotten how to plié or chasse or, worse yet, participate at all. How silly is that? Not entirely after what we’ve been through. Time flies, but muscle memory fades after 50. Of course, I stood in my old spot (upper left corner, front row) under the ceiling fan right next…