I know the region, though never done the Camino de Santiago myself, my sister did and came back with lots of storries, you will have a great time I'm sure. I'd make sure that I leave the pilgrim route every so many days and drop into some of the excellent restaurants, food in the Basque region is sublime and not as pricey as you might expect even with the Euro what it is.

Only tandem advice I can give you is to bring low enough gearing, I'd be curious to know how much will be your total climb over the 850 km. You don't have any real high points, but it is up and down the whole way, and I am sure very steep grades in places.

The cycle route and the walking route do diverge very often, there are many cyclists on the route.