Have just entered this - anybody heard of it or done it before? It's very similar to Hellrunner which I believe is held at a couple of locations in the UK. It's an "8 mile off road challenge" but it looks fab. It's held on an army base in Aldershot in Hampshire where they train the tank drivers. Waist high muddy water in early December. Can't wait!
http://www.grimchallenge.co.uk/
http://www.grimchallenge.co.uk/