A player that is passing, rushing, or receiving more during a game than they did in real life will start to degrade in performance, as well as possibly increasing their injury chances. These effects are accumulated throughout the game and as they start to go over their pace. So someone that rushed 10 times in a game will start to feel the effects if they are rush every other play for a sustained period, rather than feeling the effects as soon as they get their 11th carry. This is a gradual decline and not a steep drop off, so you do not have to worry about your starters being useless at the end of the game if they are just barely over their touches.
The "real life" numbers are based on the Stamina Ratings, rather than actual stats. This allows older players that played less than 16 games to be used for a 16 game season. It also may adjust the numbers to today's standards. Looking at modern day players will give you a good idea of what kind of numbers to expect based on Ratings.