Hmmm. I guess I'm not most people. When I walk briskly at home, on flat sidewalk, with no significant weight on my back, I walk about 5 km per hour. On the camino with backpack, on good surfaces, I average 4 km/h while walking. Then when I count in a few short breaks, etc, and bad surfaces, I find that 3 km/hour is what I can do over the day. So I'd say that 3 km/h is a better planning number.
Many people take 6 weeks to walk the
Camino Frances. You should get a guide book or look at the route online. You'll see that for most of the route, there are villages or towns every 5-10 km. The guide will indicate that most of them have pilgrim accommodation, at least from April to October. The longest stretch without accommodation is about 17 km after Carrion de los Condes. So, there is nothing to stop you from crossing Spain on the
Camino Frances as slowly as you like, without having to sleep outside.