I recently rode with a friend from Malaga to Bilbao, trying to find a route that avoided Madrid and most of the other large cities. it was a fantastic experience, cheap Easy Jet flights made it affordable and I would really recommend this part of Spain if you are looking for somewhere challenging and a little bit remote to ride in.
Most of the roads are in great condition, far better than the UK and many of them particularly in the south have a small hard shoulder almost made for bikes. The villages you pass through rarely see Cycle tourists but were friendly and supportive of what we were trying to do. We completed the journey in a week, but in hindsight taking a little longer would have been better. I've written the trip up here. www.realadventurefilm.co.uk/category/cycling
I've added a map from "Mapmyride" at the end of each day which shows our exact route. If anyone is thinking of riding this route or parts of it i would be happy to help with any information.