It is possible to step off the plane, out of the airport, and on to Jeju Olle Trail. The route is way-marked as normal. When you meet the orange line at the ocean turn right to continue into the city. It should take an hour or two to reach the Ganse Lounge (end of Route 17 / start of Route 18) where you can get your Olle stuff and have a meal at a reasonable price. Also, we recommend downloading Kakao or Naver map application.
Jeju Olle Trail Official English Guide 