The only marathon on my list (other than the putative ClassicMan) is the Marathon du Medoc - which is basically a pub crawl as it rolls past vineyards and there's fine wine at every drinks station. Like it said in Runner's World, to rush the race is like rushing a good wine: unthinkable.

(ClassicMan, for what it's worth, is an idea I keep bouncing around involving swimming the Hellespont and running a marathon into Marathon itself, with a connecting bike leg over some suitably mythic terrain.)