Expats find it challenging to sign up for races, as well as loads of other things on the Korean internet, but one person has offered to help:
Dear Ladies and Gentlemen from overseas!
Please register for Yongsangang Marathon Race online.
If the online registration system doesn't work, please e-mail your information to me!
I will help you to register.
My e-mail address is email@example.com
Director of Naju Marathon Office
I used to pass through Naju all the time when I took the bus from Gangjin county to Gwangju, and out the window it seemed small and unremarkable. However, after actually spending an afternoon there---Easter Sunday, 2007---I consider it interesting and largely underrated place to visit: take a look around the city's tourism webpage, my Flickr gallery from three Aprils ago when I visited Namsan Park and the now-defunct pear blossom festival, or my earlier posts on the city.
There are a couple other runs with approaching deadlines. The 2010 Honam International Marathon (하남국제마라톤) will take place on April 11th, with registration ending on March 19th. This one has a full marathon, from Naju into Gwangju, as well as a half, a 10K, and a 5K.
And on Sunday, April 18th is the Hampyeong Butterfly Marathon (함평나비마라톤), with full, half, 10K, and 5K runs. Registration closes on March 20th. Hampyeong county has made the butterfly its symbol, though the regionally-famous butterfly festival will not coincide with the marathon.
I mentioned these races in a January 1st post about getting back into running, where there is a collection of links and a small discussion about running in Korea. As commenter Steve points out, 10 Magazine posts race information in their regional event calendars each month, though of course I'm partial to my own updates. Nevertheless, keep an eye on their listings, because in the future I won't be able to devote as much time to this sort of thing.