Don't wear it during a mud run. Remember that this is a mud run, so you can expect to encounter a lot of the slimy brown stuff. Regular cotton clothing will most likely be stained for life, so you're better off investing in high-quality polyester or spandex gear that mud and dirt can be washed out of easily. Compression gear will be your best friend, as wet, muddy, loose-fit clothes will sag heavily off your body, making running difficult.
There are few things more addictive than mud runs. Once you do one, there's no turning back—you're a full-blown addict.
Bring your own water—for both drinking and avoiding the long lines at the washing stations.