Griffo, some things to consider:
what kind of distance can you walk with your pack day after day, with quite a lot of that walking on Tarmac?
If you've got a rough estimate, that'll give us an idea about recommending a starting point.
I walked Bilbao to Santiago in 26 days but one was a rest day and there were a few stages where I was lounging about a bit, enjoying the beach and the sights. With some military planning I reckon that 22 days is quite possible if you don't mind averaging a bit over 30km a day. My calculation includes taking the metro to Portugalete on the outskirts of Bilbao and for the last day walking the 40k from Arca to Santiago in one go. If your flight home is in the evening you'll have 23 days walking but if it's the morning then realistically only 22 if you want to attend the cathedral's pilgrim's mass and pick up your compostela.
If you think these distances are just about within your scope then an alternative to skipping to Santander would be to adopt a tactic that I saw was quite popular last year. The FEVE runs parallel to the Norte for much of the way - at least until the Galician border (I think). It was quite common to see some pilgrims hopping on and off it when they either had a very long stage to keep to their schedule or it was raining heavily or there was an industrial part they didn't fancy (like Santander-Santillana). I found the section between Bilbao and Santander to be very fine, so I suggest you take a good look at the options before taking it out
cheers, tom