Apart from that, I wonder how the long-distance running speed of animals without rider is measured. Seems like an interesting endeavor, since animals will run long distances only if they have to, which I suppose won't happen often.
Humans don't suffer from this limitation. If you're curious I suggest you read Born To Run
http://www.amazon.co.uk/Born-Run-Hidden-Ultra-Runners-Greate...
And I've often heard people complaining that training horses in hot weather is horrible because they get sweaty and all the dust sticks to them.
While I have never myself witnessed a horse sweat, I've read/heard enough references to the sweatines of horses that I assume horses do in fact ... sweat.
rid
If you ever watch a horse race or any other time a horse is working hard and fast, you'll see white foam near where the reins touch the horse's neck and and anywhere else there's friction. That's frothed up sweat.
Not only is a bipedal gait more efficient over long distances, but the fact that humans have a mostly hairless torso which is directly perpendicular to the direction they are running means two things: a maximum amount of surface area of sweaty skin is being exposed to moving air vs. body mass it has to cool. Much greater than a quadraped's.
I also wonder about the preparation of the horses. Marathon runners are, in general, fantastically and specifically well trained for their sport. Are the horses involved specifically trained for endurance racing or just generally fit horses? Are they elite endurance racers? If the horses are specifically trained, does the fact that most equine endurance races are 50 or 100 miles effect the training of the horses?