First off, it isn't John O'Groats. From Wikipedia:

John o' Groats^[1] (Taigh Iain Ghròt in Scottish Gaelic; grid reference ND380734) is a village in the Highlandcouncil area of Scotland. Part of the county of Caithness, John o' Groats is popular with tourists because it is usually regarded as the most northerly settlement of mainland Great Britain, although this is not a claim made by the inhabitants. It is, though, one end of the longest distance between two points on the British mainland, Land's End being the other. The actual most northerly point is nearby Dunnet Head.

The town takes its name from Jan de Groot, a Dutchman who obtained a grant for the ferry from the Scottish mainland to Orkney, recently acquired from Norway, from King James IV in 1496. The lower case and second space in "John o' Groats" are regarded by many as correct, as the "o'" means "of" and thus is not cognate with Irish names that begin with O'; but the name can be found with the capital and/or without the space. People from John o' Groats are known as "Groatsers".

The name John o' Groats has a particular resonance because it is often used as a starting or ending point for cycles, walks and charitable events to and from Land's End (at the extreme south-western tip of the Cornish peninsula in England). The phrase Land's End to John o' Groats (LEJOG) is frequently heard both as a literal journey (being the longest possible in Great Britain) and as a metaphor for great or all-encompassing distance, similar to the American phrase coast to coast.

Incidentally, while the signpost says 894 miles to Land's End, that is as the crow flies. To walk is around fifteen hundred miles. Not that I've walked it, but I know a man who has.