From their website:
Is Tough Mudder really the toughest event on the planet?
This depends upon your definition of tough. Sure, running 40 miles in a straight line isn’t easy, but we don’t think running 40 miles in a straight line is a real test of all-around toughness, strength, stamina, and mental grit. Tough Mudder gives participants the opportunity to test themselves in every way — all in one place and all in one day. Our relatively low prices and close proximity to major cities means that you can participate with thousands of other inspiring participants, not just a handful of endurance geeks with more time and money than sense.