May. Texas Sprint AR runs a series of one day sprints in Texas and Oklahoma including the Houston Wild Onion. The Rattlesnake Challenge several one-day races in Texas. The Great Mississippi River Series several one-day races in Illinois. Well run race. The Cal Eco Series several one-day races in California.
Dates vary by city.
Wasatch AR a 48 hour race in the roughs of the Wasatch Mountains in Utah. May NOC runs a good 30 hour race with a reputation for tricky weather. March Galyans 30 Hour a 90 mile race in the Hoosier National Forest of Indiana. May Walkabout AR a 48 hour race in Northern California. October Pathfinder Challenge a 48 hour race in the harsh beauty of Kentucky.