I guess it's one of the 'passive' skills like strength, what it does is pretty much give you more endurance so you're not going to get exhausted as easily, you'll be able to run longer etc. Also as far as I know by default everyone starts with 5 in all passive skills, feeble reduces strength for example. I think it works like that. Didn't try fitness instructor yet I prefer lumberjack atm. and there you get bonus to strength which is passive. Edit: Other thing, is it just me or does it not matter if a zombie spawned inside a building or far away from it but as long as he dies inside it he might have a key in his inventory? Because I killed about 7 zombies lured from outside into a house and ended up with 3-4 keys to my 'slaughterhouse'. It still my for me a big mystery : is Fitnes only for runnig or Swinging weapon too?