Lot’s of online posts suggest that the cycle routes out of Santander are difficult with many main roads and the east-west motorway to be avoided. We are planning just this journey in September 2012 and this page captures some of our planning.
An attractive option is to take the small ferry that runs from Santander on the west of the river to Pedrena on the east. This takes you out of the busiest side of the city onto quieter roads and a cycle route to El Astillero to the south of the city. This family-made video gives an idea of the route. Note: they are riding north to the city.
Details of the ferry crossing are available and they do take bikes. The Santander terminus is 200 metres from the Britanny Ferries Terminus – to the east.
After El Astillero and going south, you can take a green way (a disused railway track) for 34K over the plain towards the Picos de Europa and Ontaneda to the south-west.
Santander to Ontaneda by this route looks to be a relaxed 40-50 kilometre day.
Query: can you cross the river back to El Astillero?