Do it on your own, whenever, then stop by the bike rental shack for your medal. It's the nice rubber medal that's
Castaway Cay specific, not the "winner" ones from the trivia. They can also show you the map where to run, basically start at the shack, out to the loop to the tower, out to Serenity Bay, the loop to the tower again, back to the shack. Info current as of late Nov 2021.