The famous Chesil beach in Dorset. It is a shingle spit eighteen miles long. During World War II it was used for testing bouncing bombs.
Walking along it is tiring, to say the least. I walked a couple of miles along it and when I came off (and my legs had readjusted to walking on solid ground, rather than deep, sinking shingle) I realised my legs were aching and tired. I think the ultimate running race in the world might be a race along it.