Many thanks for that. I'll have a look at the map. I rode from Toledo to Avila a few years ago and if I remember rightly it was a good ride and not that far.
Mike
I'm a little loathe to offer routes because some of these I first did over fifteen years ago. More recent re-visits have shown that EU money chucked at Spain in the intervening years has been spent on straightening a lot of what were once great roads.
These were great roads, I can't be sure that they all still are.
Bilbao to Burgos: C6318 via Balmaseda and Bercedo to the N232, you then have to turn north a little way to the N623 and south to Burgos. The good thing about picking-up the N623 here is that it is at a "Mirador" (a viewpoint and they are usually worth stopping to take in the views).
If you then take the quickest route to Toledo via Madrid, it's not very scenic. However if you turn west onto the N110 for Segovia, you will be on what was a lovely stretch of road with the Guadarrama mountain range on your left.
Crossing the Guadarrama south from Segovia is good but may be time consuming if you need to be in Toledo by a certain date.
Segovia is fabulous. Worth it for the Roman aqueduct alone.
If you want to take in Avila, carry-on from Segovia on the N110 (west), it runs into the N501.
Don't bother with Avila except from the outside if you are short of time, the best views of the walled city are from outside the walls. (If you have seen the film, The Pride and the Passion with Sinatra, Sophia Loren and Cary Grant where they drag a huge cannon cross-country, you will recognise Avila instantly).
Turn south at Avila onto the N403. Nice route, the first stretch is the most scenic with another Mirador en route. The N403 will take you on to Toledo.
I haven't been on any of these routes since the early 90's. In 2005 I found that some roads like that from Salamanca to Caceres had been straightened in part. What's even more frustrating is that the Spanish often leave the old interesting road in view from the new one but without any way of getting to it. The old sections are just abandoned to nature.