The standard required trot pace for a VSE on marathon is 9km per hour, or 6 minutes and 40 seconds per km. For most of the minis, large or small, you have to hold them back to an easy pace in order to avoid coming in too fast over a 5-10km course. A lot of people are pushing to use the pony times which are 12km per hour as they think the minis can easily trot that speed. I tend to agree although with my 33.25" mini I'd rather go ten or eleven km per hour as his legs are a lot shorter than the 37"er's! LOL.
Right now in my area the minis do not do the walk section so I couldn't tell you how fast the expected marching pace there is. I do know an excited mini can keep up with the big guys on a trail ride though so I would guess 4MPH also. That's what I was taught the "standard" walk speed was for a big horse.
Leia
Edited to add: Remember, 15 miles is a long way for an unshod horse. Watch their feet closely if you intend to do this all in one push, bring Easy Boots, and condition condition condition before trying it. The best way to find out how long it will take is to simply have the horse go a measured mile at a comfortable pace (be that trot or walk for the individual horse) and time it then do the math factoring in the fact they will slow over distance, need breaks to rest, etc.
