Norty hit it right on the head, IMHO the human body as it stands right now is just not equipped to handle the muscle mass and violence. Unfortunately this is what you get with Shannahan running game. I would be willing to bet that the average for one of the Shanny RBs of the past is around three years in the system.
What needs to happen is for us to kick a 4th or 5th over to Buffalo for Breida who is currently slowly suffocating buried in the depth chart. He is also injury prone but at least he knows the system