Recently completed the Jeju Island loop. We had originally planned to finish it in 4 days but the weather took a turn for the worst.
The Island loop is very cyclist friendly and easy to navigate with clear signs and paths. There are cafes and convenience stores every couple of km so its easy to stop and appreciate the views.
We were lucky enough to catch the Cherry Blossoms in bloom which made some paths really beautiful to ride along. We also opted to stay in pensions (guest houses) which I found were much better than hotels and cheaper too.
by Umz