Hi Petra,
To add to my previous reply to your posting, you wanted to know about traffic. The worst traffic that you will encounter is at the big cities, such as Santander and Gijon. To avoid this take the FEVE trains. The station at Santander is less than 5 minutes walk from the Brittany Ferry terminal.
You can for example get the train to Requejada, every 30 minutes at about only 3 euros. This gets you to the outskirts of the extended built up area around Santander. From Requejada it is a very short ride to join the Camino del Norte on the CA 131, en route to Santillana del Mar.
The area around Gijon and Aviles is also very built up and industrialised. We cycled from the pilgrim hostel at San Estaban De Leces (west of Ribadesella) to Gijon, a long day ride. We arrived late evening but could not find the hostal in Gijon. Every one that we asked sent us in a different direction, as darkness decended! However, if lost, ask a policeman. The hostal was behind the police station!
Next morning we caught the FEVE train to Cudillero to avoid the conurbations and well worth the train journey. Again, frequent trains and very cheap! Have a look at
http://www.feve.es/es/ This is in spanish but if you google FEVE you can get an English translation. Lastly I would suggest that you start learning some spanish or improving what you already know!
Cycling around Gijon in the dark and repeatedly asking directions, getting food, hostel accommodation, and train tickets, etc, soon highlights language deficiencies!
So put this high up your list of priorities- tengo que aprender espanol!
A Picos Rider.