Looking for a flexible endurance challenge? The Next Loop Endurance Series at Cliff Cave Park in St. Louis features a 3.1-mile trail loop where you start fresh every hour—run as many loops as you want and stop whenever you're ready, with no penalties or DNF marks against you. Your score is simply the combined time of all the loops you complete, making it perfect for runners of any experience level, whether you're tackling your first 10K or you're a seasoned ultra runner.