Horse speeds
One item of fact that you may wish to consider in your argument about how fast one can travel on horseback: the Pony Express, durring their brief existence, mantained an average speed of 20 MPH across the American West. This was riding flat out as fast as the terrain and weather allowed, changing both horses and riders at regular intervals in order to keep them fresh and rested.
I'd suggest that as an absolute maximum sustainable rate of travel, so long as you've got replacement horses, or mearas. Everything else would be much slower.
And honestly speaking, 3-5 MPH as a rate of speed for travel by horse is not that unrealistic. That was the speed of travel of comerce and news and correspondence before the advent of rail and auto. While horses are capable of traveling much faster than humans for a time, if you are traveling and you need to keep your horse in shape to continue traveling the next day, and the day after that, and so on, you have to husband its strength. So you travel slower; at a Walk (3 MPH) or a Trot (5 MPH). While you are certainly able to Gallop a horse at a speed of 11 MPH or more (as suggested by the Pony Express's average speed), the poor animal is not going to be able to keep that up for eight or more hours of travel a day, day after day; it's going to keel over and die.
There's some information out there on the Web, if you hunt long enough, dealing with travel speed in the pre-mechanized age, and it really wasn't much faster than a steady walking pace.
-Chris Landmark
"Was entstanden ist, das muss vergehen. Was vergangen, auferstehn." -Klopstock & Mahler
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