Have you been charged by a bison? I have!
I've been running for the past decade, and one location for a number of my races is Antelope Island State Park, in the Great Salt Lake, Utah. The Park hosts the spring Buffalo Run (25K, 50K, 50 mile, 100 mile races), the fall Antelope Island Fall Classic 50K, and the fall Mountain View Trail Half Marathon (13.1 miles). Although an ultra is anything over marathon in length, Ultrasignup.org registers shorter distances. For the most part, they tend to be trail runs. That means you dodge around rocks and roots, and the trails are not straight. For a road marathon, there is probably an aid table every 2 miles, with a porta-potty as well. For the 50K, I think there were 4 aid stations in total, and no porta-potties. In fact, on Antelope Island, trees are rare. A private potty break means finding a boulder, or a gully, or a small sagebrush. My ultra ranking, from Ultrasignup.org, currently is 73.61%, and my age rank is 94.35%. My fastest times were 1:44:03 (half marathon), 2:38;26 (25K)