As Sagalouts sort of says, you don't really need to decide on a destination. You may not have a good idea of how far you will walk each day until you are on the road. You may want to explore some town or building, or take a rest on a sandy beach, or have a day off. And why not? It's a very beautiful part of the world.
Whereever you end up you will be able to get a bus or a Feve train back to Santander.
Instead of a finishing place, have you considered choosing your starting place?
We travelled 'back' to Guernica, which was where we had ended up the year before, from Santander, using a bus to Bilbao, then the little basque railway to Guernica.
We passed through Santander and walked on to San Vicente de la Barquera, where we stopped and caught a bus back to Santander for our booked sailing back to Portsmouth. Our blog (link below) might help a little, although I have not yet gone back and posted extra photos.
Regarding route, there seems to be quite a lot of choice about the Norte. Really you don't need to worry too much, all will become clear on the ground. Call in at the Tourist Office from time to time and collect some of the many leaflets which show routes and list albergues. These will also be available usually at the albergues. The Tourist Information tends to be specific to each region (The Basque country, Cantabria etc) so make sure you stock up when you cross a border.
I doubt you will have a problem with albergues being too full in September. Weather wise, I would expect sun and warmth, but also rain - this IS an Atlantic coast after all.
Finally, I found this website good for routes, mapos, information about towns along the way and places to stay:
http://www.mundicamino.com/ingles/
if you think we could help with any specific query, do PM us.
enjoy your planning