The official Jeju Olle listing gives Trail 3 (Onpyeong–Pyoseon) as 20.9 km, about 6–7 hours, rated challenging. Plan separate transport to the starting point and from the finish.
Check the official Jeju Olle site for diversions and alerts. Bring water, plan for wind and rain, and verify crossings, bathrooms and the last bus. Times vary with pace.