If you'd like to try out a mud festival in Korea but don't want to deal with the overcrowding, the drunks, and the pervy photographers that are associated the famous one in Boryeong, there are a couple other options in the Jeolla provinces.
Yeonggwang county will be holding its 3rd annual Mud Marathon Festival (머드마라톤대회축제) on July 30th and 31st. The actual "mud runs" will take place the morning of Sunday the 31st (registration closed July 15th, but hey I gave you plenty of advance notice), though other performances and activities are scheduled, including a mud slide, mud sleds, and tug-of-war. A Korean-language program is available on the website. County buses run from the bus terminal in Yeonggwang-eup to Yeomsan-myeon and Baekbaui Beach (백바위해수욕장).
And check out Jeollabuk-do's Gochang county the next weekend for a similar set of events (전국갯벌스포츠축제). Hat tip to the Gwangju News editor for that one.