Michael-- the distance progression shouldn't have anything to do with it, and a) older horses are most of the basis for figure making, and b) younger horses get stronger and faster as the year goes on, which complicates things. But the question of distance itself is interesting. Problem is I don't know how you would prove it.

AS a practical matter, most horses run on dirt before they run on grass, so you use those figures as part of your decision making process.