Well, that's five years where we don't know what he was doing. A lot changes when you've been feeding off of the demon magic. It literally changes a person and how they look at life. And to say he has no prior knowledge of horses is definitely not true.
Yeah that was my point - it's not that there's absolutely no way that it could have happened (though I do feel that based on the in-universe rules set out at that point it would've probably taken more time than that) - the problem is that we don't get to see the character development. I was expecting to read the story of how the passionate farm boy became the cold-blooded and purposeful demon-killer, but the book skipped the most important parts of that path.
Instead of a continuous narrative, we got the origin story and the realisation of the hero's strength, but not the part in between. The first analogy that springs to mind is Spiderman - it'd be like watching him live happily with his aunt and uncle up until he gets bitten by the spider, and then skipping to him kicking some supervillain ass. The most interesting part - the transition from a normal person into a superhero - is absent.
A lot of that happened offscreen, but when the um... You know, the messenger guy. Anyway, when he was training him, he had to learn about horses and riding in order to be a proper messenger. Also, considering he grew up in a hamlet, he had plenty of time to deal with horses there. The messenger training is where he got the majority of his experience though.
It's vaguely plausible, but it's still odd that it's never mentioned before he suddenly has the perfect horse; besides, the whole "it took me less than five years to breed the perfect warhorse" thing is the bigger problem. It leaves a hell of a lot of questions unanswered. Prime horses would have been incredibly well guarded, and he shunned human contact as much as possible - so where did he get the horses from? He was supposedly living in the wilderness and learning to fight the demons properly - so how did he find the time to protect the horses, train them, and so on? It's a serious undertaking, and it's just completely ignored.
Not saying it's a detail that breaks the story for me, but it's an example of the kind of thing that left me scratching my head.
Also, the gender roles dramatically change as the series goes on. You meet one of the most powerful women ever, whose only forced to stand on the sidelines because of the Krasian religion. That starts to change as the series goes on. Also, the duke's grandmother plays a pretty powerful role as well, but again is held back from showing it publicly because of how people feel. Then there's Leesha of course. You don't get to see it in book one, but she becomes super awesome. She's mentioned a whole lot when people discuss strong female characters that aren't stereotypical.
Interesting, guess I'll find out myself if/when I continue the series. Sounds like you might be concentrating on the existence of strong female characters - which I felt was better than some, all things considered - rather than the nonsensical cultural customs regarding gender roles which just didn't feel like they fit the situation in the setting, but I could easily be wrong about that!