Hi - how did you get on? I waddled in under just under 21 hours. Managed to trip over a tree root - worst injury being minor discolation of top joint of little finger - doh - only Bear could do more damage to hands rather feet on an ultra !
Will be be there waddling off at the 8.40 start. 2 litres def. OTT. Plenty of snacks on the way so you do not need to carry too much extra fuel. Route is fairly straightforward and you'll be given a fairly detailed map at registration. Will wear normal running shoes 2 sizes larger than normal with Scholl footpads under forefoot, a pair of liners and walking socks. The path is quite hard and stoney in parts and as it's flat there is a greater risk of blisters as the pressure is always on the same area. Just hope there is not a heavy mist - on one event (not this one) managed to walk right by a check point as it was completely obscured.
The XNRG Extreme Energy events are incredibly beginner friendly. In spite of the tough guy image given by their name it is a family run organisation that encourages everyone. The check points stay open as long as there is anyone on the course. There is transport from and to the nearest station included in the cost + baggage transfer for multi days. There are several events through the year but you might like the Chiltern Challenge - 31 miles pick up point Princes Risborough - 11th July. The Cotswold Challenge which is 57 miles over 2 days 12/13th September - you can pick a one day option. Otherwise I would suggest you look at their website - sorry my linky thing has been broken for ages.