Its a charity walk isn't it?? i did it a few years ago - a road walk of 40miles was quite straightforward, if a little sore on the old feet. It took me just under 12 hours, and for my trouble i was given a voucher worth four pints a beer and a lovely young lady massaged my legs - well worth the effort!!! Oh, and all monies went to a good cause.
P.S. You will need a personal stereo - The Ramones blasting in my ears meant I could at least walk at a good pace!!