Average career for an NFL running back is 3 years. That is at the age of 22 or 23 just out of college.
If he gets injured, NFL teams will consider him too old to recover and they will move on.
http://en.wikipedia.org/wiki/Brian_Banks_%28American_football%29
Banks signed with the Falcons on April 3, 2013, and will participate in Atlanta’s offseason workouts, which begin on April 22. He is competing for a spot on the team at inside linebacker. On August 8, 2013, Banks made his NFL debut in a preseason game against the Cincinnati Bengals, where he picked up two tackles.
He will be making tackles, and hopefully a lot of them..........