On this page of the Assassin's Creed Valhalla guide, you will find a detailed walkthrough of the Wincestre region side quest: Mildberg the Miracle Legs.
Reward for completing the mission: 1200 XP.
In the place marked on the image above, you will meet a girl called Mildberg. The girl claims to have received a gift directly from God - feet as fast as the wind.
Mildberg will suggest a race - agree and run after her. When you catch up or overtake her, the girl will stop and admit that she never thought she would be defeated, and the mission will end.
The girl will start the race by circling a nearby column, which you can use to your advantage. Instead of running directly after her, run along the road in front of you - you will immediately overtake her, and the mission will end immediately.