Tough Mudder: London South

If you think triathlons are for wimps and the Three Peaks have peaked, this 10-12-mile run with 20-25 military-style obstacles designed by the British Special Forces may be just the endurance challenge you seek. Unusually, a number of the obstacles can't be completed solo, making this as much a teamwork challenge as a race, and participants have to battle all manner of obstacles, including fire, ice, water, electricity and yes, mud. Lots of it. And you don't even get a medal for completing the course – how hardcore is that? You can choose your start time when you register, and registration closes on June 2.