Great question. Running fast incorporates a range of muscles, and these are the same muscles responsible for slow running, hiking, and even walking:
Calves
Quads
Hamstrings
Glutes
Feet
Core
You will feel the different emphasis on certain muscles when you push the pace. For example, when you do sprints or tempo runs, you’ll likely feel increased tension on your hamstrings. Training these muscles to endure heavier loads will prepare you for faster efforts.