Hi, everyone, or should I say, mates,
Alex, By my count, you've got about 30 walking days. I think that Irun to Oviedo is about 500 km and Oviedo to Santiago is about 300 km. My computer calculator tells me that's almost 27 km per day. That seems well within the realm of possible for people on the other side of the middle - age divide than I am.

I'm 57 and try to plan for about a 24 km average, with a rest day or two thrown in for good measure. On the norte, we spent a day in Llanes and a day in Oviedo. We were totally unrushed and made it in about 35 -36 days I believe. Of course you have to deal with the fact that the albergues aren't going to be at perfect 27 km intervals, but I think it sounds reasonable and do-able for you.
My walking partner and I met in Bilbao and spent our "get over jet lag" day there before taking the bus to Irun. So we had a chance to visit the Guggenheim. I'm not much of a modern art connoisseur, but I thought the exhibits inside were for the most part depressing downers. Sunflowers drenched in lead for example. But the building, wow, it is amazing and we really enjoyed walking around it, walking on the river across from it, climbing up to the top of the bridge next to it, so I am definitely glad we went. The entrance fee is pretty steep, I think around 12E (?). But the old style museum across the road, the Museo de Bellas Artes, is more my speed, with its Sorolla, El Greco, Ribera, etc. And it has a very nice coffee shop as I remember. I was definitely glad we spent a day in Bilbao, the old casco historico is really nice as is the neighborhood across the river with stately buildings from, I think, early 1900s.
My Spanish is good and that helps a lot, but I have met tons of pilgrims who speak very little or no Spanish. I can't help with a phrase book recommendation, you might want to post a question about that in a more general forum like "Equipment Questions" or whichever topic seems more closely related. I will say that Spaniards, like people everywhere, appreciate it when people try to communicate in their language and if you're armed with a phrase book and gestures, I don't think you'll have a problem, because people will help you out.
I'm no beer snob, in fact I rarely drink it at home, but there are few things more satisfying than sitting outdoors in a cafe, freshly showered after a long walk and watching the world go by with a Spanish beer. Whether it's great beer or not, I don't know, but I guarantee it will taste great to you. If Jan is reading this post, he can corroborate this (and maybe give us a few brands that he likes) -- I remember many times when he arrived at our destination before I did, and he and Erna were already sitting there with a beer even before the shower!
And thanks, Sil, for clearing up what was being visited in Oviedo. I definitely need to do some more reading -- am I wrong about there being a very old stone Christ figure in the cathedral that is important to the Camino's history, too?
Alex, I think you will find fewer places on the norte that serve "menu del peregrino" but all Spanish restaurants serve a "menu del dia" but usually only at lunch. We did eat a lot of seafood and fish on the Norte, with my all time favorites being Las Arenillas in Islares and the hotel across the street from the pension in El Peral. All the tourist towns on the coast are of course loaded with restaurants and seafood, but the places like the two I mentioned, in more out of the way places, stick out in my mind for the extremely high quality and low prices.
Ok, one more comment before I get back to work (my coffee break is ending.....), on the detours. I agree with Annette that the E-9 detour on the day into Llanes is one of the most beautiful walks anywhere. In fact, there are several other E-9 detours that I mentioned in my description of my etapas on the post with my pictures. I took every E-9 detour I could find, they always go back to the Camino. I think Johnny Walker makes a good point when he notes that the marked route is always changing and so maybe it's silly to worry about "authenticity." In my own experience, I've walked the Frances 3 or 4 times now, and each time I go back, there are always re-routings, either because of highway construction or because of neighbors complaining about the hoards (at least that was the explanation given to me when I asked in Estella about why the way in had changed so dramatically).
Sorry to keep going on and on, you can see this is definitely work avoidance on my part! Laurie