the most difficult Jeju Olle path

The Jeju Olle trail is 425 km long and has twenty-some segments, each with its own length and difficulty rating (called a nanido/난이도 in Korean). I was thinking of flying out to Jeju to see what the "difficult" level was like, but then I found the following hiking video* about what is supposedly the trail's most difficult segment (18-1, I think?):

It's important to keep in mind that the Olle path is for hiking, not biking, although I imagine (as one commenter pointed out) that it's likely the hiking and biking paths coincide for a good part of the trail's length. That said, the above video shows a lot of the trail isn't bikable. The segment seems to have been about 20 km in length, and it took the hikers all day to do it, although after having done five or more peaks along the way, they didn't seem overly tired (then again, they all appeared to be in much better shape than me!). I got the impression that the friends started their hike rather late in the day, which is why the video ends at night. I would probably start around 5 or 5:30 in the morning, assuming Course 18-1 is even part of the route I'll be walking. (I'm not even seeing it on my Jeju app.)

Whether I end up doing this particular segment or not, I now have a much better idea of what counts as "difficult" on the Jeju trail. I think I'll likely end up tired as hell during these segments, but that's fine. That's what hiking is all about. One sinister thing, though: in the video, they note that it's November 29 as they're hiking, and they're all in short sleeves for much of the hike despite remarking that the weather is really fall-like. I'll be hiking there from late September to mid-October, so it's going to be warm. The hikers did also moan a lot about all the mountains and steep slopes, so I've got that to look forward to. Jeju is even farther south than Busan; I imagine it takes a while for the weather to really start cooling down. This looks to be a beautiful route, though. I look forward to it.

ADDENDUM: Course 18-1 is entirely located on Chuja Island, north of Jeju Island proper. The hikers did keep mentioning Chuja island, and I wondered whether it was one of the little islands near Jeju that is connected to the big island by a bridge. It is not. It's in the big water, all by itself, about halfway between Jeju and the mainland. That said, the video is still informative about what counts as a "difficult" trail by Olle-path standards.

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*The video's title says, "Most Difficult Olle Path Adventure." The white-font dialogue says: "How many mountains did you say...?" "There are five..."