Greetings,
I did the reverse journey on Dec 12th from Moncofa which is 40k north of Valencia (I had to go slightly southish to start with, look at map you will see what I mean)
This was our first trip on that route and we are not yet what I would call experienced in matters of judging time/speed etc. in Spain.
Our journey took us (and I'm reversing it for you, Bilbao AP68 to Vitoria, Miranda de Ebro, Logorno and Zaragoza. At Zaragoza we took the A23 to Teruel and Valencia. This road is very new and there is still an unfinished section around the town of Daroca.)
We planned to stay in Olite in northern Spain (just below Pamplona) HOWEVER,
the journey was on very good roads with scarcely no single carriageway. We arrived at Olite at 2.30pm in the afternoon. The following day it was a short three hours to Bilbao.
Had I been doing the same journey again I would do it in one go, especially at the start of a holiday.
I accept however that we are all different and even with the Olite stop, I would put in a nights accomodation on the return 'just in case' of problems.
As it was, when we got to the port of Bilbao, the ship was in, as were about 200 people who had had to sit there for four days because the weather had been that bad in the Bay that one complete round trip had been cancelled.
Have a good holiday.