Why does it have to be time played to give you better stats. It really should take on a Elder Scrolls type where; Running gives you levels to... running (duh) Using medical items makes you better at using only those specific items Catch a cold? Surviving it should in reality give you a stronger immune system to combat it later on Kill a deer? Get more meat out of it because you learn how to cut it up better. Make a coat out of it or hell a dagger out of bone to stab humans... err zombies. The leveling process should be quick maybe three deer kills max... the odds of finding or living longer than that should be low... this is the apocalypse. Running maybe three full days. Endurance could actually stretch for quite a while. But anyway apply that however you want. It should not just be a log in timed duration to level. That would be stupid for a sandbox with this potential.