The specific question came up in the context of the Disney Marathon, which is actually on Sunday, and the previous day on Shabbos is a half marathon. (They have a combo award for those who do both). Agreed, one should not do the half marathon on Shabbos. However, the question I came up with, since the race is done mostly on roads between the various theme parks, where no one really lives, so on top of everything else, it seems likely that the Techum issue would be there.
But the land is all owned by Disney, and fenced, and protected. How does Techum work in such a case? You have a LARGE plot of land, large enough to contain a city, then a migrash then a techum, and still be entirely on private property.
Does Techum still apply if it is all on private land? Especially if fenced in completely (Which I do not know to be true of Disney, but assume it likely).