Is running with that speed, continuously, not in intervals, considered as hiit? And does it develop fast or slow twitch muscles, or both?
HIIT training is a form of workout where you alternate between about 30-60 second periods of intense exercise and less intense exercise, so running at a constant speed would not be considered HIIT. Running usually falls under the category of cardio.
Slow twitch muscles are developed when doing endurance exercise and fast twitch muscles are developed when doing exercise that requires quick, powerful movements. I would personally say that running falls in the category of slow twitch muscles, as you are constantly exercising your legs over an extended period of time.