Two orcs
Officially better than you, according to PoN
Also, horses will slow you down in mountains or wetlands, since you have to take detours to find routes to accommodate them, and you would probably need to lead them much of the time instead of riding them. A lot of hills are also pretty tough, as are forests with a lot of deadfall and/or undergrowth. Horses are plains animals and don't do well with steep slopes, deep mud, concealed uneven ground or lots of things to trip on.
From what I've seen of professional knights riding in armor (either plate or mail) doesn't seem overly tiring - for the rider! I always figured the movement penalty in rough terrain was due to forced detours rather than every step being X% heavier, but it´s an interesting point that horses would be even more limited in viable routes - in that case certain terrain should reduce horses to pack animals and even exacerbate the movement penalty (or introduce a risk of losing the mount to a leg injury). If we want a really granular experience I´d say that traversing a swamp hex deals 1d6 damage to each horse - 0 or worse hit points means it breaks a leg! You can still do it, but after 18 miles of swamp the average horse is dead (and high hp horses become sought after! in ACKS this would tremendously upvalue the Animal Husbandry proficiency (Healing proficiency for animals) and I´d also let them look a horse in the mouth to determine their exact hp before buying).
This takes advantage of the predictability of hit points, you can introduce risk and resource management into wilderness travel - a shorter route might risk your mount. This could also be used for when you ride them too hard, go further at the cost of damage to your mount. A strong mount can be ridden much harder, but will need more time to recuperate (this might have strange interactions with Cure Light Wounds, clerics making parties a lot faster. On the other hand, you'll want one mount per party member so the CLW can´t cover them all so unless you are a very cleric heavy party it won´t break even).
The Fantastic blog Wandering Gamist has a long post on D&D logistics rooted in a book on Wild West settler (a good read in itself, it´s essentially an adventure manual with topics ranging from how to drive your horses, how to make a fortified camp, how to identify different Indians and when to be wary). The Wandering Gamist: ACKS: Simple(r) Logistics