The Ebro has its source about 50km south of Santander and runs south to the Med. There is now a foot/ cyclepath the whole 1,000 km. The route is the GR99 and a guidebook is available. I have had experience of cycling in Spain on the pilgrim route to Santiago de Compostella and found the parts on roads not too enjoyable owing to traffic, and off-road not too cycle-friendly.
Thought people might be interested. I may give it a go later in the year but will avoid June, July, August owing to excessive heat.