"Aerobics Is Good for Your Sex Life" by Wyvern (an6445@anon.penet.fi). This story was reposted as part of the collection from the "Vault," under the title aerobics.sty. However, I have had terrible luck getting complete stories from that series, and so I cannot recommend getting other stories from that source. Maybe that service will improve. It sounds like a good idea. Here's a review of the one story I did download.

Imagine that you are an overweight, middle-aged man who has decided to get his act together by doing some exercise before his heart stops functioning completely. I know this will be a stretch for most male readers of these reviews, who are almost unanimously young, virile, and amorous hunks. But for the sake of discussion, assume that you were such a person and you signed up for aerobics at the local YMCA and the cute young aerobics instructor made you an offer you couldn't resist. To wit, as the lawyers say: if the party of the first part would meet his weight goals by a designated date, that party would be invited to the rustic cabin of the party of the second part for a weekend of fun and frolic as a reward for his good behavior. Not a bad idea! Needless to say, the protagonist meets his goals and becomes a veritable stud. Let's hear it for positive reinforcement! The author presents the story very well, and the sex (which he refers to as vanilla) is actually pretty hot. I thoroughly enjoyed this story. Maybe YMCA's would be a lot more popular if they had more programs of this kind.