Mary-Ann can be found in Pacific Bluffs and is only available during the daytime hours. After talking to her a bit, you'll have to race her on foot. Note that Mary-Ann is quite fast and if you don't have high stamina by now, you might lose, although you don't need to win to finish the mission.
If you do want to win, which is the only requirement for the challenges, then make sure to cut corners as much as possible. She will smoke you on the stairs, but you can cheat a little bit and use the thin area next to the stairs to keep a full sprint going. Once you reach the final stretch and are in front of her, use your body to block her if she's trying to pass you.
Gold medal walkthrough: