walk :-) the camino del northe,if you have the time,it is ok if you arive at any hour on every day by car ,plane,train,bike or .......

Rent a care and choose one of the thousands nice towns in Cantabria or Asturias to spend the night.. Comillas, Santillana, San Vicente de la Barquera, Llanes, Lastres, Ribadesella ...you won't regret it.. or speed up and make it to Gijón in a couple of hours... and enjoy Gijón nightlife..

If I were you, I´d rent a car and sleep on the way.

I agree with Antoine: public transportation in the Green Spain is not so fast or regular. It's easy and fast to travel from the North southwards, but not acrosss the Northern regions. I'd rent a car and sleep on the way. The landscape is very beautiful, the food great in all the regions you will cross and you're very likely to find a nice and affordable accomodation.

Hiring a car will probably help you saving time (it's a 500 km trip), but it's not absolutely necessary: the best way would be to go Santander-Gijón or Santader-Oviedo (by bus), and then take another bus to Santiago...

to be honnest: hire a car !!!