Running a marathon is hard, lets not pretend otherwise, but to really test your mettle in the world of running these days, the only way to go is ultra. An ultra distance race is anything over a standard marathon in length and they usually start at 50km.
If you’re thinking about improving your bragging status in the office here’s a list of some of the toughest trail races in the UK. By the way, all of the following give you qualifying points for the daddy of trail races; The Ultra Trail du Mont Blanc.