Three week Camino del Norte - which part is best?

The shorter distances are possible after Santander which we did for my benefit. Walking from Bareda to Llanes or Ribadasella is a possibility. Take the cercania from Santander to Bareda and walk into Santillana (approx 8kms) then the stages can be made as long/short as you need. We used the CSJ printed guide and also Gronze website which has distances and up to date accommodation lists. The FEVE can be a help on the latter section and we took a taxi into Unquera from the cafe in Pesues (See gronze etapa 14)
A mix of road and track with some amazing scenery and also choice of accommodation etc. Probably possible also to start east of Santander using the same idea, but we have not walked that part and the section between Santander and Bareda is very much road walking through an industrial area.

I would favour starting at Irun, which is practical from the point of view of flying from UK to Biarritz and getting bus to Hendaye. I started from Irun at 1pm and arrived at the delightful albergue at Pasajes de San Juan at 5pm. Hard walking but if you take your time should be ok for your wife. From then on the coastal route is delightful and there is plenty of accommodation. Some sections may be demanding but shorter days are quite practical on this camino. My mistake was to go in July when there were many young people who were using it as a holiday rather than a pilgrimage, resulting in overcrowded and noisy nights.