I've said it before and I'll say it again. When I watch the "tape" from last year McFadden looks ok to me like he's running hard and getting what's there (he doesn't finish like Murray, but he's definitely faster than Murray). Even though the metrics are terrible. I have no answer for this disconnect and interested to see what McFadden can do behind a good offensive line.
That stipulated, can anyone think of a really good second act for a running back in McFadden's situation and particularly his age. It just doesn't seem to happen much. O.J. Anderson went to the Giants at the end of his career and helped them some. Dorsett had a couple of decent seasons with the Broncos as I recall but nothing special. Marshall Faulk had a great second act, but I don't think anyone was questioning his ability at that point; they just wanted to go in a different direction (similar with Dickerson). Herschel Walker had some decent years in Philly as well.
On the other hand there is a long trail of once great running backs that proved the second time that they were really done: Franco, Eddie George, Earl Campbell, Dorsett to some extent, LT, etc. Point is running backs don't seem to age well and it may be a lot to expect a rebirth in a career at this point.