Cyn's never been shod. And they're about four weeks since trim. You're right, I think if you're going to ride like that all the time they should be shod. I don't think there's a horse that wouldn't get sore if they did that all the time.
Those little golf-ball sized rocks in the path are the worst part. They don't last a long time, but the rocky places we do have, they can't avoid. I think the places that are graveled are easy going, but will wear them down just like sandpaper. We'd shoe Cyn if we did that alot. But once we do, we can likely never go back :~)
Meaning the 15 - or further... She does fine on doing the 8 once or twice a week :~)