My daughter (18) is in her gap year and we may well be there, definitely Spain North to South, but route yet to be decided, will be a Santander south start or an Gijon start vaguely following the Ruta de la Plata.
However, I have done your proposed route before, but starting from Bilbao then onto Santander before heading Soutrh. I have cycled all over Spain and IMO, being so close to it's most beautiful region, you will be missing out. Cantabria is beautiful, but Asturias staggeringly so, they are worth seeing even if the trip takes you longer, or you have to take a train to catch up.
I usually do 40-45 leisurely miles a day in Spain, I like to look around, I also like to enjoy the late nights, then I can't get up in the morning..........