The one thing I notice with Aiden is that he berates himself when things don't go right, to the point that his head literally goes down. My guess is that this is more of an issue. Speaking generally, how can you tell if it's physical fitness or not have your head in the game, that's stopping a player from tracking back?
I've seen Aiden track back very effectively on some occasions, it's just that sometimes he doesn't. That suggests attitude/mentality to me, not fitness.
But here I am again - what do I know.
Bottom line is in the games I've seen, he hasn't been that bad - more of a team issue. And he's yet to reach the form we saw a couple of seasons ago.