Leith Hill Tower is definitely the best place for spectators - you get to see the runners twice, there are good view and they do tea and cakes from the little shop in the tower!
Along with two stalwart marshals, Nick and Mike, I ran the third quarter this afternoon, taping and painting. By gad it was tough. It is extremely muddy. You could certainly get away with doing it in trainers (which is what I was wearing), but trail shoes would probably help if you have them.
Good luck all!