Funnily enough I was advising a friend about doing North South Spain just this w/end. Slightly confused by your route description. Santander to Santiago to Salamanca to Sevilla?
I've done the regions around Santander on several occasions and also did a comprehensive tour of Andalucia starting in Seville, finishing in Granada. The minor roads (and do plan via minor roads) are generally very good for cycling and indeed would recommend you use them at all times if you want a relaxing trip - detailed maps are a must (1:250,000 ideally) - and many roads in Spain have been renewed in the last 5 to 6 years which makes for superb cycling surfaces, although I found it can take the edge of the sense of adventure/remoteness cycling on such pristine tarmac.
You've given a very wide brief indeed, so best do some research as suggested by Andybg and come back with more specific questions.
Another great tour-planning resource is Trento bike pages :
http://www.trentobike.org/ I'm sure there'll be plenty of trips around Spain on there to inspire you.
The one recommendation I can make is to be sure to come the Sierra de Morena as you approach Sevilla, a beautiful area of semi-wilderness, now protected by a series of national parks, if you like that sort of thing.
