Thanks. Increasing consensus that wrong time of year. Might think againI cycled Lisbon to UK in May 2014. I followed the Camino routes. Granted I was carrying all my kit so the daily distance you are talking about are well above what I would consider. But Spain was high, lumpy, bitting winds and at certain stages heavy rain, hailstones and bitterly cold. FYI.
I echo what @Somerset said (bolded bit above). I rode from Valencia to Santander at the end of March 2018, and while it was a fantastic ride it was also quite tough for the same reasons. Coming out of Santander you are faced with a wall of mountains (the Cantabrian range) to go through or over, then the terrain remains at 800m or above for much of the way, with snow on the highest ground at that time of year.I cycled Lisbon to UK in May 2014. I followed the Camino routes. Granted I was carrying all my kit so the daily distance you are talking about are well above what I would consider. But Spain was high, lumpy, bitting winds and at certain stages heavy rain, hailstones and bitterly cold. FYI.
Thanks. Increasing consensus that wrong time of year. Might think again
time of year sounds dodgy, but if you can go later you can adapt the route I took from France https://www.crazyguyonabike.com/doc/page/?o=2sf&page_id=303217&v=9h cut across to Valladolid and you have the N630/620? all the way for hundreds of km. A big wide empty old road as all the traffic goes on the free motorway - my blog has all the details.
Always a challenge for planned tours: you have to allow for horrendous headwind even if it doesn't materialise, whether that's by restricting mileage so you can complete a day by plodding at 6mph or whatever, or by bailing out onto a train, or by cancelling/rebooking accommodation (probably at a cost).A very relaxing road to follow and make good daily distance on. But, the OP's target is way too ambitious, particularly as the 2 times I've gone that way it's been into horrendous head winds.