As to rest days, even though I didn't plan any, in the end I think it worked out best for me. I started on the Norte with long stages and the first week had some very long elevation gains. What I ended up doing was shortening two stages:
One, the first week, after a grueling walk to Guernica, I thought there was no way I could continue the next day and was prepared to take a taxi. The next morning, I woke up feeling better, took a closer look at the route and realized that I could take a taxi to the mid-point (to skip the woods and elevation gain, and have a gentle downhill through a paved route.
The second one also followed a very long walk the day to Santander. The guidebook indicated that the first portion was mostly through commercial/industrial area and recommended taking a train to by-pass, which I did (there's also a coastal route, but it's long, not well marked apparently and I knew I was too tired for that).
The reason I mention this is for two reasons - my body felt better walking the shorter route rather than not walking, and also of all the various towns I walked through over 42 days, not many would I have wanted to spend an extra day in - just not that much to see, for me anyway. I did walk to Finisterre and Muxia and spent an extra day in Finisterre, and was happy to do so and spent a happy day walking the paths around the peninsula. So maybe bear that in mind with your planning if that sounds more like what might be better for you.
Also, if you are a fast walker, I would encourage you to slow down a bit from time to time, savor what you're experiencing, take some pictures, look backwards to experience your view from a different perspective. Unless you just want a fast physical challenge or are trying to catch a bed in an albergue, there's no reason to rush. At the same time, everyone has their own pace, I'm sure you'll find yours!
Buen Camino
