I saw a couple of discussions about airport gyms, one of them from 2005.
I recently found the following website: http://www.airportgyms.com/ - might be useful - anyone knows if it is up to date?

I am starting this thread partially because I am sure I will remember "there was this website about airport health clubs ... what was its name ..." in a year or so. Now it is documented here. Do not take this as a guarantee about quality/accuracy of the website.

It does sound like a good idea for airport hotels (or maybe even a franchise concept) to actively market one or two hour access to a gym on or near an airport for a fee. Throw in a couple of miles to draw in frequent flyers (status qualifying because "the airlines want to stimulate a healthy lifestyle") - and a smart entrepreneur may have found a nice little niche.