Agreed it's very subjective using a rule like Naismith's and there are so many many variables however for high level planning only...why not? It's fun to plan and then see how far off on is!
So many variables...heat, fatigue, trail conditions (cobblestones, hot asphalt, tree roots after a rainstorm on a steep descent, beautiful forest path, bushwalking or something like it), lack of sleep due to snoring, great news from home, sad news from home, rain and more rain, brilliant sunshine and the company of a fellow pilgrim who becomes a new friend and time just flies, wind, injuries, meeting a massive cow blocking the trail in Switzerland, a scary cow on the The Puy route with bloody scary horns and taking a short cut and falling in a manure patch and then accidentally touching an electrified fence etc. etc. etc. Oh and did I mention age...I could easily do this at 30 when I ran a few marathons...now at 62 could I even walk a half marathon? And the big one getting lost again and again on less traveled Camino routes...wandering around aimlessly and chewing up the km's?
Anyway that's the fun and challenge of walking Camino's...so many variables...nothing is predictable and the enjoyment of simply walking can't be beat. Looking forward to a new adventure on the Camino Primitive!
Guy