I guess I was lucky getting my long distance training on the 3 day 100's
here in the East where the entire route has markers every five miles and
every mile for the last five or ten. After a while you get a real "feel" on
the distance. In the old days (when courses were measured by car rather
than a wheel), the trail master would ask several veteran riders how well
the course was measured. It was uncanny -- if the course were half a mile
or more over or under, the old timers would all have mentally noted the
variance! If a marker was placed wrong (sometimes necessary for various
reasons), the trail master hears about it!!
For training purposes, once I've determined a ball park MPH for whichever
horse I'm riding, I keep logs based on Time X MPH. Works for me.