Note, dear writers and readers, that the title of this piece is not "How fast can I go?" but "How fast should I go?" If you can discern why that is crucial at this point, congrats! You've probably given thought to the fact that your poor equine beasties are not machines. If you're scratching your head right now, then read on.
I recently reviewed a decent book that had a good story and, to be fair, was not focused around horsey things, but since it used horses as basic transportation I naturally took note of how well the author did or didn't portray them. Alas, in this case, the author fell straight into the Hollywood trap of the bottomless stamina of the saddle horse. His character had a powerful warhorse (naturally) and never... walked... it... a... step. The poor beast never even trotted in the book. The guy cantered or galloped it everywhere, for miles and miles on end. And the horse never seemed to get tired!
When I pointed this out to the author, he was surprised. To his credit, he was perfectly willing to learn, not being a horse person himself. He said that he had assumed the canter would be easier on the rider and so that's why his character cantered everywhere.
Oh, my. He's right in that the canter is infinitely easier to ride for most people than a trot, especially if you never learned to post (rise from the saddle every other stride in rhythm to the horse's trot). But if you want to get anywhere on horseback and have a horse at the end, the speed is not about the rider but about the horse. Back when shank's mare and the old gray mare were your two basic choices for overland travel, the beast of choice was an ambler, a horse or mule with an easy gait like a pace or what evolved into the running walk or slow gait. Breeds like the Tennessee Walking Horse, American Saddlebred, Paso Fino and Missouri Foxtrotter all evolved through conscious search for easy-gaited animals who could offer a comfortable ride for long days in the saddle. For long journeys or inspections of ye old plantation, comfort was wonderful. But rider comfort is secondary in any situation where the horseman depends on his mount. Getting where you're going with a live, healthy beast is more important than whether you get jounced a little on the way.
Running your horse everywhere gets you exactly as far as you get before the horse drops dead under you, which may or may not be your actual destination. For well-conditioned horses, that's a lot farther than the average backyard beast can manage, but there is still that hard limit. Expecting to ride 50 miles a day endlessly is just so not in the cards. Twenty miles in the mountains is a long day (trust me). For well-conditioned horses and riders on decent roads in not particularly difficult terrain, 30 is a good average and still leaves you time every evening to care for your animals. After that, it gets iffy, because rain, snow, mud, availability of grass, group size, and a hundred other factors start to play into the equation. But speed is the issue here, so let's stick to the conscious decision of the rider, who is, after all, the guy controlling how fast his horse is being made to move.
Extraordinary circumstances, such as a military forced march, may necessitate pushing the stock way past what they can handle--and the commander has to accept the subsequent losses as horses break down and fall out along the wayside. This leaves part of your command on foot and the rest with blown, exhausted horses unable to do much at the far end of the march. In normal circumstances, the pace is always set predicated on what the animals can handle for the day, not on how fast Mr. Imperious Rider wants to get there. It really doesn't matter if the road is good and the grass hasn't all been grazed to nothing by all the travelers who've been this way ahead of you. Your horse is simply incapable of running everywhere under your weight without breaks. Even the Pony Express went flat out for just a few miles, then changed horses and sped on again.
A horse's best traveling gait (and easiest on the rider) is a good long walk. Next is a trot, which many horses can keep up for quite a long time. The trot is, however, much harder to ride. The western "jog", which is a vastly slower trot, is easier to sit but, for me at least, induces a screaming sideache pretty quickly and it's not at all speedy. In a natural trot the horse flows freely and carries himself better, which means his back and legs hold up better over longer distances. The rider can help him enormously by not sitting in the saddle like a sack of potatoes, rump banging away against the saddle seat at every stride. Posting puts the rider's weight in the stirrups, evenly distributed across the horse's back through the saddle tree, every other stride. He "sits" just for a second on the second stride, then rises again. This greatly improves the horse's stamina and it's a lot easier on the rider as well. A rider who learns to post does it naturally and can keep it up pretty much as long as the horse can.
If you're in a hurry, trot a couple of miles, walk a while, trot some more, walk, repeat. You can cover enormous amounts of ground this way, much more than if you tried to gallop the whole distance. Case in point: My father once rode his incomparable Saddlebred, Fox, 42 miles in 28 hours in the Cascade Mountains to find help for my grandfather, whose horse had gone over backward on him and broken his pelvis. Fox, being a Saddlebred, had a wonderful natural extended trot, which Dad used to effect on every foot of ground that wasn't straight up and down, and walked the rest to give Fox a breather. He did it in a western saddle, pretty much standing in the stirrups during the trot phases.
So. If your fictional riders and horses are on the move, ask yourself where they're going and when they have to be there. There is no excuse to needlessly push the horses, and if your heroes are in a hurry for a reason, then remember that their poor ponies are not machines. It doesn't matter if the princess is going to be executed at dawn; put your hero a plausible distance away if you are going to have him run his horse into the ground getting there to save her. I don't care how good the beast is, he's not going to cover 200 miles in a night. Overnight forced marches of 50 miles are on record, however. And even then they weren't running their beasts, but likely traveling a good, steady six miles an hour for 10-12 hours, with rest breaks. For the sake of your horse and your groaning audience, give your poor pooped pony a rest now and then.
One good little overview of historical foot and cavalry "standard" march rates is here along with some "notable" marches made by both.
Until next time! I'll be working on Seaborn in between, which I am pleased to report progresseth apace. If you want to catch up on the series, Firedancer and Windrider are both available in print and as ebooks.