I done the Playtex Moonwalk Marathon in 2002....it's basically a marathon (full 26.2 miles) but you can walk/power-walk it. You can opt to do the half marathon if you would prefer. You start at midnight and go around london finishing back where you started, it's a great walk for breast cancer....plus you get a free wonderbra and t-shirt (and a medal at the end).
http://www.walkthewalk.org/Home
They do them all over the UK now, and one in New York (expensive to od that one!) and one in Nepal or somewhere similar. I done the London one.