I whole-heartedly disagree with your first statement, at least with the thought that he has a few more productive seasons in him. When running backs fall off, they fall off QUICK. LT had a terrible season on a per carry basis. He's not going to get any better. Dude is done.
I look at corey dillion as my example. he was 29-30 when the pats got him, and they got 2 decent seasons out of him. I don't see him rushing for more than 1000 yards, but I think you can bring him in as a #2 or change of pace back. you can still get some milage out of his legs and he can still contribute, it just won't be the LT of old.