Take a look at this site, it may help you decide:
http://caminodesantiago.consumer.es/los-caminos-de-santiago/del-norte/
I would suggest you walk from Bilbao to Santander, and if you are able to cover more ground than that aim for Santiallana del Mar, hang around this beautiful old town, and visit Altamira. Mind you the bit between Santander and Santillana is a bit off putting, but you could jump ot and make up those extra two days by walking from Santillana to San Vicente de la Barquera which is a nice walk and interesting early Gaudi architecture plus the old university.
It's easy from any of these places to get back to Santader (intercity bus a few times a day) and from Santander to Oviedo the bus take a little bit more than 2 hours.