Hi, chris,
40 km a day will put you in the "elite walkers" group -- you will only meet a few people who keep up that pace day after day. I've met some over the years, and they tend to be from Western Europe, male, wiry, and unstoppable (and usually are in their mid 40s or older).
The "good walkers" tend to walk in the high 20s to low 30s a day, I'd say, with the occasional high 30s or low 40s if absolutely necessary.
I think most people (at least your average fit middle aged person) find that mid 20s is a good average, but of course everyone is different.
I don't mean to suggest this is a scientific breakdown, but just the generalizations I make after 10 or 11 caminos. But I will say that 40 kms a day, day after day, will generally take a real toll on the body that's not used to that pace and amount of pounding on the feet. If you can be flexible, I think the best advice would be to try it out and see how it goes without over-doing it. When my son was 19 he walked the Camino, and he was at the time very fit, but he was a typical know-it-all teenage male. He almost had to stop after his first four days. He met a couple of very experienced middle aged Germans who invited him to come along for their 40 kms a day ride. Fortunately, my son stopped and recouped and let his body repair itself. He recognizes now that he never would have made it to Santiago at that pace. The key thing is to just listen to your body, it will speak out loud and clear if 40 kms is too high a daily pace.
The Camino Primitivo has a fair amount of ascent and descent, but nothing an average-ly fit person can't easily walk. I would also say the average number of days from Oviedo to Santiago runs from a low of 10 to 15 or 16. You will love the Primitivo! I'm going to walk it again this summer, but I will be a few weeks before you. Buen camino, Laurie