How long can Gore (soon to be 31) keep running?

Daily 49ers blog for NBC Bay Area. By some standards, Frank Gore slowed down a bit in the second half of 2013; by others, he was just as strong. But how long can the 49ers' standout running back keep going? http://www.nbcbayarea.com/news/sports/Gore-Still-Staying-Ahead-of-Father-Time-245202771.html