I've always thought the only way for the airport fitness club thing to work is for a well known gym brand (Equinox, Gold's, etc.) to partner with an Airline Club to put locations in their bigger flagship lounges (e.g., AA LAX lounge, DL JFK lounge, etc.).

Could add a tier of membership to the Airline Club pricing and visitor's could also buy a 1 time pass. It allows you to take advantage of the big footprint many airline lounges have, the showers many already have, and provides a natural co-branding / co-promotion opportunity. Plus, it gives the highest likelihood of attracting the Type A frequent flier type who actually sees value in working out while traveling (e.g., they travel frequently enough that they seek out a gym whenever they have time as opposed to the twice a year flier who can't fathom the idea of working out at the airport).