Hi, John,
I just walked the Aragones this summer and thought it was very well marked, though other pilgrims have posted some disagreement with that assessment. We never got lost, never had a doubt about the route. But that's going "forward". I am personally pretty hopeless when it comes to finding my own way, so I would never try this route backwards. Unlike the GRs in France, the Caminos in Spain are only marked in the westerly direction. On a camino like the Frances, walking backward is much less of a problem because you are likely to have people coming towards you who will show you the way. Not on the Aragones, which is sparsely travelled and getting less popular by the minute I hear (down another 20% this year, but that's another story, can't for the life of me understand why).
If I were going to do it, I would definitely take a GPS and then there would be no worries. But you may not like gadgets, so if you are a great map reader, go for it! Buen camino, Laurie