So, to mix things up a bit, I would second the suggestion of the Primitivo, but since that gives you fewer kms than you want to walk, and since you like mountains, why not start in Leon, or a few days before Leon, and then in Leon take the Camino del Salvador for 4 - 5 days over the mountains that separate Leon from Asturias. That takes you to Oviedo, where you can start the Camino Primitivo. There is plenty of information/advice on here about both of those routes, but in a nutshell -- they are stunning!
As a ballpark, I walked the Primitivo in 11 days and the Salvador in 4 (and I'm 60, which means nearly twice your age, OMG). That means you would have some cushion and some extra time to sightsee if you like that sort of thing -- Leon, Oviedo and Lugo are all "worth a visit." Or start a few days before Leon to get warmed up on the flatlands.
Time of year may make a difference, because I remember that a few years ago, Kevin and Kari on this forum started the Camino in October a few weeks after me and decided to move down to the Frances because of all the cold/inclement weather on the Primitivo. I started from Leon around September 20.
Anyway, if this appeals to you, let us know because there are lots of us here with experience on these routes. Buen camino a todos, Laurie