The only route that links those two points is the Norte, and you may struggle finding accomodations only 20 km.
Bilbao to Portugalete: 19 km inland, 10 via the river but oh such an ugly walk along the industrial zone following the subway route.
Portugalete to Pobena: 14 km
Portugalete to Castro Uriales: 13 km, no options between Portugalete and Castro.
Castro to liendo/Hazas: about 25km, with only other option at km 12 in Pontarron
If you opt for Pontarron then next stop could be Laredo 9km past Liendo.
Liendo to Noja : 20 km
Noja to Guemes: 14 km
Guemes to Santander : 11km
You will have to play around with distances, short and longer days in my opinion. The Eroski website can also help plan.