Ever wondered what the difference between red meat and white meat is?
Animals like cows and sheep are herd animals. Their survival strategy for attacks by predators is to be able to out-run them. Not run faster, just run for longer than the predator. This means that their muscles can be saturated with hemoglobin for longer; they can't fill the muscle with blood as quickly as an ambush predator so they have to build up to their top speed, but neither does the blood leave the muscle quickly, so they can maintain that speed for longer.
Predators and some other animals on the other hand don't need endurance, they need the ability to sprint from a standing start. These animals have muscles that are designed to be flooded with oxygenating blood very fast, providing massive amounts of energy quickly, but they drain just as fast meaning that the speed cannot be maintained. Very much a 'light that burns twice as bright lasts half as long' kind of thing.
So; sprinters are more likely to be predatory in their nature so that's a start. The first thing that would be needed for their adaptation is longer legs.
Longer legs mean larger muscles, but also larger 'levers' in the form of the bones. That means for every contraction the muscles make (especially quads and hamstrings), the further you go distance wise with more or less the same energy. The tradeoff here is that your hominid would lose a little agility in this so they can charge an enemy line, but they're less likely to be able to dodge fast spears or swords. They would be taller though, so perhaps they'd be able to see further into the enemy ranks and anticipate attacks better, making up for the reduced capacity to react. Balance would be another weakness in this case because of the long legs and lack of agility when running.
If their arms were also longer and they had strong upper body strength, they could wield larger swords and maces, making them quite dangerous in melee situations against smaller foes.
Your hominids would have evolved from ambush predators who work in groups to take down larger prey. They would have larger reserves of blood in their system to maintain a powerful sprint even for a short distance, and that would mean larger lungs to fuel the energy conversions. This probably means faster metabolisms (they'd have to eat more than a normal human, probably by a lot) and they'd tire easily after their short stint in combat.
They'd probably live a shorter lifespan than a conventional human. Faster metabolism is one thing, but using all that energy in a single burst would put a massive strain on their systems, just as it does with Cheetahs for instance. You can also see the longer, more spindly legs on these creatures by comparison to other cats.
For the record, horses are endurance runners, especially when they are fully mature (say around 6 years old). That means that they have good speed, but not the acceleration that a Cheetah is capable of (although they can run for a very long time).
So; I'd be using your hominids as cavalry hunters. Ambush them, kill the horses (or preferably the rider) and then get out. They would make great neutralisers of enemy cavalry ranks if deployed correctly, whereas against a line of pikemen and other infantry, they would be less effective.
In short, you use your cavalry against infantry, and your hominids against cavalry, and your infantry against hominids. Very much a scissors, paper, rock scenario in my humble opinion.
How would these hominids evolve? They're ambush hunters, pure and simple. They band together to take down larger prey, but use speed and weapons instead of strategy. They rely on overwhelming their prey rather than herding it into a difficult terrain or trap like the neoliths. I'd imagine that these hominids would evolve in large sweeping savannas, where the terrain can't be used to advantage and where stealth and teamwork are the answer to hunting. Much like modern lions.
Obviously I don't have hard data available, but with the right metabolism, blood and hemoglobin volumes, lung capacity, limb length, etc. you should be able to get your hominid up to around 80 Kph; fast enough to just outrun a horse over a short distance (again based on the Cheetah). This though would inhibit many of the other traits that you want for melee combat, which is why I suspect they would only fill niche tactical roles.
