I don't think you understand how insanely impossible it would be for this bird to exist practically speaking. Dropping 5mph does nothing to change that, nor do "Super Strong Legs" Compensate for foliage density and the need for agility.
First, bird based beings are generally not heavy, and therefore do not need "Super Strong Legs" to carry itself with ease. The ostrich (another plains creature and not dissimilar to your own flightless bird) runs with the aided effort of it's wings, using the wings in a flapping motion to propel itself along even as it runs. That being said: Lets assume your creatures legs are somehow super strong and can just ignore the ridiculously thick foliage of the jungle (an adaptation that is shared with no other creature in history except perhaps a few limited cases like the elephant and the rhino, depending on how you view it). Your bird is still going to die from trees. Take you on a motorcycle. Now, say you were cruising along a highway at 45mph. Suddenly, trees. Trees everywhere. You are in a rainforest. I am guessing you aren't dumb enough to run into a tree, however unless you slow your cycle way down, you are going to die in this rainforest. It's not about intelligence level, it's about the ability to 1. Process the information of literally hundreds of trees all hurtling at you at 45 mph 2. Registering a safe path which may not even exist and may require a complete course adjustment 3. Acting with enough agility that you can shift your 45 mph mass several times completely adjusting course constantly throughout your run, regularly fighting your own body's momentum to avoid every tree, all while never once slowing down or dropping pace.
Having super strong legs with a lot of muscle mass, actually makes it worse in this case, as that is just more momentum that needs to be shifted. Horses and Cheetah could not accomplish the agility necessary to move at such a rapid pace through a dense jungle, that's why they live in plains environments.
I'm not saying your bird has to be totally realistic. We are in a fantasy world after all. I am just saying that A] You are really stretching the realm of imagination with this animal and B] A bird capable of traveling 45mph, and predatory (meaning it help in combat) gives the Kha a huge advantage, and in theory they could win virtually any war through cavalry alone. An Orc might be able to face a charging Gy'waka, but a Kha on it's back with a sturdy spear would make such a feat impossible. And since your bird has such ridiculously strong legs and crazy unreasonable agility, traditional trapping methods used to box in charging cavalry in our RL wars of the past, are virtually useless against your bird. For example, there was a trap where spearmen would stand to either side of an opening against a charging horse. The horse, not wanting to be skewered, goes between the two spearmen groups. It is then met by a wall of shields, halting it's advance, and surrounded. The method was used to take down countless charging chariots in the past. Your bird cloud literally stop and turn on a dime, never missing a beat and never coming close to being trapped.