How is position/player playing time determined for the pre-season games?

Under your "misc game planning" screen there is an option for when to pull your starters during preseason.

You will want to play your starters long enough to get them into football shape, but the more they play the higher their risk of injury. In fact, you will see lots of minor injuries during preseason; once your players get into football shape their injuries will reduce considerably.

Thanks JDB, that's easy enough.