Hi, rellrog,
If you have time, consider starting in Irun or Hendaya, because the day from there into San Sebastián is quite nice. I think you'll find that the norte is extremely well marked and well traveled, and you should have lots of company in September. Irun to Bilbao is about a week.
From Bilbao to the
Camino Frances, are you planning to walk? The so-called Camino Olvidado or Viejo Camino goes from Bilbao to Villafranca del Bierzo in 18 stages. I am going to walk this route in the summer with another forum member and we plan to have a guidebook available when we return, so if you are interested in walking this route, we should have plenty of information for you before Sept. I already have a word document with a translation of a basic guide and can send that to you if you are interested.
There is a section of the forum devoted to this Camino, but the main thread with the most resources is this one.
http://www.caminodesantiago.me/comm...ejo-bilbao-villafranca-del-bierzo-2014.21807/
As you know, arriving in Villafranca would then put you a day's walk from your friends in O Cebreiro, so it seems to be well suited to your plans.
Buen camino, Laurie