Well since this is important issue, I did some googling...did you know there is an actual website called horseloversmath.com?
Anyway, this below is not from that site but StephenBrown.com says:
According to Goose Wing Ranch, horses have four “gaits” or types of speed – I’m sure they are all familiar terms: walk, trot, canter, and gallop.
Walking horse avg 4 mph
Trots avg about 8 mph
Canter about 14 mph
Gallop is 28 mph avg.
(*a human’s average walking speed is 3.27 mph.)
In my own running days long ago now, I preferred a jog coz you could cover much ground for very long time...so yup looks like the horses trot around 8 mph! (which they would need to do to keep warm anyway, walking horses might freeze!) So more fancy math here, thirty-one hours in that open December sleigh!