I remember when Brian Mitchell was asked, after he filled in as the 3rd QB in a Redskins game... "Are you going to be a QB like you were in college?". Brian said, "No way. I'm a RB. You cannot bulk up to be a RB and throw the ball well."
Nixon needs to work out like a RB, think like RB, train like a RB, and play RB. He can still come in and do the wildcat short yardage stuff. If he's relegated to QB workouts to start this season, he will not be an effective RB.