The Goliath race event is unlocked through expanding the main "Mexico!" (Mainstage) Festival site in Horizon Adventure.
The race event is 34 miles/54.5km long and is found on the East Coast of the map, next to the "Mexico 21/Playa Azul" text box.
Edit: for this achievement, you have to complete and win the main, default Goliath and not an EventLab event as these won't count towards the unlock.