A rule of thumb has no complete description. As I said, not all miles
are equal (some are fast, some are slow, some are hilly, some are
flat, some are sandy, rocky, muddy, etc). I also agree with Stephanie
that individual horses require individual approaches.
It seems, though, that at least 3 successful endurance riders (Truman,
Stephanie, and myself) condition far less than 52 miles a week.
(whose horse's winter vacation is now getting longer than planned
because of the cold and snow)