Hi, walkmag,
I came from a different continent, but after a lot of searching, I found that the best option for me was Chicago-Madrid-Bilbao and Santiago-Madrid-Chicago. Adding the internal flights from Madrid to Bilbao and Santiago to Madrid rarely add more than $100 to my round trip air fare, and that is a lot less expensive and the cost of adding on a flight on another airlines, a train or bus. (You can find these fares by doing a "multi city" search on whatever website you use).
Last summer when I walked the Olvidado, I was in Bilbao by noon, having arrived in Madrid early in the morning on an overnight flight. I took the airport bus into town and had the whole day to walk around and do the pre-camino chores like buying phone, sending some stuff to Santiago, etc. Got my first stamp at the cathedral and then off bright and early the next morning. Bilbao is well worth a day or two, but I had been there on several occasions so I settled for a walk out to the Guggenheim and some time in the nice inner neighborhoods. Buen camino, Laurie