Hi John,
I see that you have already received some useful advice. I note that you are now planning to ride from Santander. I did the Camino del Norte from there with 3 others in 2005. We cycled all the way on the roads using the CSJ guide for this route. At £5 it is worth every penny. In addition to the walking route it also details the route for cyclists. I can confirm, no problems with traffic although you cannot avoid it, in for example Santander and Gijon/Aviles.
I see that you will use Michelin maps, so did we. They are hard to beat for quality and detail. Their new Zoom series at 1:150,000 are even better. You will need 3 sheets, No.141 Galicia, N0.142 Asturias and No.143 cantabrian Coast. If you live in the UK try Stanfords the map shop
http://www.stanfords.co.uk/ I tried Amazon but they did not have all 3 in stock.
Last time we flew from Uk to Santander and flew back from Santiago with Ryanair. Four of us are going again this year (also in July) but going by ferry to Santander, and return by ferry from Santander. I have given up on flying after problems taking bikes on planes over the past few years. Cheap flights are not cheap, add in very busy airports, security checks, bike dismantling and bagging, damage to bikes, bikes which did not arrive, and compare that to walking on and off a ferry with your bike.
One last point about in town traffic. The FEVE railway station is 5 min walk from the ferry terminal. In 2010 when cycling in the Picos mountains we took the train a few stops to get out of Santander and its suburbs. Trains every 30 minutes and very cheap. Similarly cycle into Gijon and get the FEVE train to past Aviles. This avoids the industrial areas and city traffic of these two adjacent towns.
I am in a cycling club and am doing a write ups about our training for this year's camino, and planning for the ride. You might find some of this useful or interesting, see
http://anerleybc.org/planning-a-cycling-adventure/
A Picos Rider