Adrian Peterson is not the normal running back. Him at 60% is better than 99% of the running backs in their absolute prime. He has no qb, everybody knows he's getting the rock and he plows people over like they're freshmen in highschool. We won't see another rb like him in a long time.
Did you know that Adrian Peterson's lowest YPC came when the Vikings had, far and away, their best passing game with Favre, Rice and Harvin?
He's special. He's already an all time great. And I agree, we probably won't see another like him in a while But at the same time, I don't think he's immune from what other RBs of the past were not immune from either-- time and age.
Especially considering how much physical punishment he dishes out and takes. His game is based mostly on his explosiveness (power, speed, acceleration). Guys like that just don't hold up that long.
Personally, I think he's Dickerson incarnate. Dickerson had his last great season at 29. It's very possible that Adrian Peterson holds up longer, but at the same time I wouldn't be willing to take on his 12MM a year contract for the long haul based on what I've seen from running backs in the past.