I went up to Seoul for this years ultra marathon. Only did the 50km course which was enough for me. 
The course is a basically flat course along the AnYangCheon River down to and about 6km along the Hangang to the BangHwa Bridge, there is a little climb and sescent about 1 km from the start. I don’t know about the fast riders but all us slow riders stop for a drink and something to eat at the turn under the bridge.
The weather was good, bit cold at the 8am start but there was no wind to speak of and by the afternoon it was quite pleasant. We started in groups of four at one minute intervals.
There were 46 entrants in the 100km race. Jeong JeongGuk won in a time of 4 hours 9 minutes. Of the 46 starters 35 finished in under 6 hours. 
The 50km was won by Park YunSeong in 1 hour 57 minutes. Yours truly finished in 3 hours 27minutes… But at least that was an hour faster than when I last rode the race, about 3 years ago on my 29. 33 of the 48 finishers came in under 4 hours.
And yes, there were lots of prizes thanks to the Juggling Shop/UDC Korea.
A few general comments:
The second place in the 100km went to Lee YeongGyeong riding a non-geared 36, in 4 hours 16 minutes. Only one 100km rider was not on a 36, and that was a 29c.
Much more variation in the 50km race. There were 12 32" wheels, the fastest coming in in 12th place. Jang MinSeop took the cake though, he got around the course on his 24 (looked like a freestyle, not a thin racing wheel) in 3 hours 8 minutes.
As with all longer distance events, you do need to train! I had done a reasonable amount of riding in October. I rode the GeumGang River bicycle trail mostly over the Chuseok holiday, finishing it 3 weeks later. Had to give up due to a sore backside I did go and run the Jeonju Ultra 100K on the 14th which was just a distraction… The Sunday before I had done a three and a half hour ride with a friend, and I struggled to keep up all the way, it was windy too. This proved to be worthwhile as I hadn’t done any fast riding at all and in a race most of us do start out going faster than usual, which has a consequence on the return trip… I got down to the turn in about 1:35, so I slowed down a lot towards the finish 
I’ll put a few links to videos on the video page.
Cheers
