Mud run courses usually have water and fueling stations throughout, but you don't always know what they'll have or what they'll run out of. By the end of the course, you'll be dying to refuel, and you don't want to get light-headed with all that running and obstacle dominating.
Pack Gatorade, Clif Bars, a water bottle and whatever else you may want after the race. For those who might need to refuel during the race, some mudrunners pack energy chews by double-wrapping them in sandwich bags and pinning them inside their clothes.