The "Head-to-Head" is a run between the southernmost (Mizen Head) and northernmost (Malin Head) points of Ireland. The exact route is up to the runner, but it is roughly 550km (mostly on roads). Eoin Keith discusses the route and history here.

According to the Facebook-page of Richard McChesney, he finished a full circumnavigation of the M25 motorway around London (158 miles/254 km) in 43 hours and 58 minutes (April 28-30, 2017?), doing this without sitting down once, doing it in a supported style. There was a live feed of his attempt available, and he's posted a write up on his blog.

Info provided by Wouter Huitzing (prior to his 2019 FKT run):

The Pieterpad is the most famous and longest* hiking trail in The Netherlands, spanning an official distance of 498 km. It starts in the north in Pieterburen, and goes to the south (St. Pietersberg), where a continuation on the European trail GR5 is possible. Of course the reversed direction is possible too. The trail is mostly hiked by older people, walking one or two days during the weekend, but a few fast attempts have been made.

The Wales Coast Path runs 870 miles along the entire coast of Wales.