There are national/international sportspersons around. I guess they, and any other fitness fanatics from pre IOT, remain pretty fit afterwards too.
Most though I reckon (including me!) sit around in offices or crewrooms drinking too much coffee and drink too much in the bar or down route, buy new bigger blues trousers every few years and find that, when they come to get married or get stitched for some formal duty, their No1s don't fit quite like they did at Cranwell.
The only requirement for physical rather than medical fitness is to pass an annual test. You have to run up and down the gym in time to the music and do some sit-ups and press-ups. The length of the music and the no of 'ups' get smaller with age. When you're really old like me you can ride a bike instead of running. If you're aircrew you have to do some swimming too.