I did this last year.
As csb, ferry to Santander. I then took the train up to Reinosa but you could just ride, then about 14 days to teh sea with a couple of days off. Stayed in a combination of 1* hotels(hostals) and campsites. Just watch for the mosquitos in the delta!
Then I rode and trained up to Barcelona for a flight home, but if you’ve a couple of days spare you could get the train back via Paris then the tunnel. If all the bike spaces are taken you can still take your bike in a box for free. I just asked in a bike shop if I could have a box.
I managed to navigate the whole route with the guide book and a couple of Michelin maps. It’s really easy to follow the markers most of the time. There were two passes that I had to carry the bike over, one small section where the path had collapsed and a wee section that the land owner obviously didn’t want the path across his land, but they were just wee sections (with a bit of Spanish I would have known to avoid them!). Overall the majority was rideable and enjoyable. Some ace technical sections in there too along with quite a lot of gravel roads. Ace weather in the main too.
Good food and wine too 😀