It is crazy how much the shelf life of running backs has gone down over the last 10-15 years. Running backs seem to get old over night. I mean LT is a perfect example. It doesn't seem too long ago when there was talk of him seriously challenging Emmitt's rushing record. Then bam, he started having health issues out of the blue.
I think a lot of it has to do is you now have LBs who are 260-270 running 4.5's and 300 pound linemen running in the 4's. The game is now full of bigger, stronger, faster athletes and you now have a 210 pound back running 4.3, coliding with a LB who is 270 running 4.5 head on and a 300 pound linemen running 4.8 hitting him from the side 20 plus times a game! OUCH!
I think it is why you have running back by committee now and unless you have a freak like AP or Trent Richardson, running backs aren't selected high in the draft anymore.