Me and 3 mates are taking part in this walk; http://www.fourinns.org.uk/ It's 40 miles across the peak district, with a 24hr time limit. The thing is the furthest I've walked in a single stint is about 16 miles. That was part of a 3 day walk, where overall we did about 32 miles in the peak district. What should I expect? Any particular distance barriers where its hard to keep going? My fitness is pretty average, although I go to the gym 3 times a week, I do very little cardio.