No, nothing to do with cocaine. Everything to do with horses.
A crack rider was someone who rode extremely well, stayed in the saddle well, probably jumped very well, and might, in 1812, if he were in the military, either be in the cavalry, be a courier, or even ride intelligence. That is to say, he’d very likely spend perhaps 20 hours per day in the saddle.
So, how did he do that? And didn’t he get saddle sores?
It’s helpful to remember that someone who rides like this is a highly trained athelete–although perhaps we don’t think of them as such these days. Riders in general have very strong leg muscles, obviously, but also very strong back and shoulder muscles. And like any athelete, a crack rider’s stamina and tenacity far exceed those of the non-athelete. So while he would certainly be exhausted by the long days, it wouldn’t have the same impact as on an occasional rider–he’d still be able to walk when he wasn’t on horseback.
Second, if one is riding fast, it’s unlikely that one is sitting heavily in the saddle like a sack of potatoes. (Although that is how Napoleon is alleged to have sat a horse…) One is more likely to be up and slightly forward out of the saddle–it’s easier on the bum, and it inhibits the horse’s movement and speed less.
But yes, saddle sores. They are a real thing. Exactly like a rug burn, only in places where you really don’t want a rug burn.
Even a small saddle sore may quite literally keep the sufferer from sitting down for a week. Bad ones–I cannot even imagine the agony of that–and yes, one might be bleeding through the backside of one’s breeches…
For the prevention of this, there were sheepskins or other soft coverings for the saddles of those who’d need them. The only problem with sheepskin is that if it gets soaked in a downpour, it takes a good day or so to dry out–and the splosh splosh splosh of sitting on a dripping sheepskin while trotting is truly disgustingly unpleasant.