I cant remember the route but did somthing similar back in 1996, from Santandare in northern Spain down to Barcelona on the Med, only got lost once in Zaragosa, if I remember right it was just over 500 miles.
I stuck to the autopistas these are toll roads, nothing like the uk motorway network alot less trafic the only reason for using these motorways was that i had to get to the port of barcelona to catch the ferry to Mahon Menorca.
It was a great trip, if I were you I would look up the way on google maps, good luck and have fun.