Agree with Janet.
Santander practical for experienced walker & pilgrim, as yourself.
I got Irun to Leces in 14 days walking. With pretty good weather (But some leg problems.)
Fair amount of asphalt road in places, stunningly beautiful in others.
A good start is to Orio, 42 kms but great Albergue (private) & Deba is only around 28 km the next day.
Monastery stay at Zernazza then highly recommended. Your host, Monk Gregory charming gentleman.
After that is a difficult call, we ended up at Morga Pass youth hostel, 2 km off route down the main road. OK, but not memorable and little alternative, as I recall.
Leaves you a relatively easy walk to Bilbao, which I found difficult to get out of & wasted considerable time & effort. (Make sure you know where you are going !
Next stop easier 25km to Castro Urdiles.
Laredo 38km (or Santona 43km)
with following day Guemes (a must) around 26 from Santona, or 31 from Laredo
This way 10 days to Santander, and a shortish 25kms to get here.
Continuation readily possible and enjoyable. Could even make Villaviciosa, if good weather and if you're in fine form but I wouldn't count on it. Best to be pretty relaxed about exactly where you finish. (Bus to Santander returns thro most these places.)
Day 11 to Santillana at 34 km & very nice, somewhat touristy
Day 12 to \serdio/Estrada is around 38 kms new Albergue, food in nearby bar.
Day 13 to Llanes (or Poo where good private Albergue). Llanes seems an albergue beds hotspot. But 37 km to Poo albergue, is near to sea/beach not obvious.
Maybe that's where to stop and sort out return.
Enjoy, Buen Camino,
Valdis.