I just read on the Facebook-page of Richard McChesney that he finished a full circumnavigation of the M25 motorway around London (158 miles/254 km) in 43 hours and 58 minutes, 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.
As far as I could find, there are only two other documented attempts at a full circumnavigation. First there is the previous attempt of Richard McChesney, which he consideres as a failed attempt because he slept in between. This supported attempt took a total of 86 1/2 hours. The other attempt is made by Alastair Humphreys, documented in his book Microadventures, which he describes as an adventure in "the most boring place I could think of". This attempt was together with his friend Rob (last name not mentioned in the book), took about a week, was during the winter, self-supported and in no way an FKT-attempt.
Last Edit: May 15, 2017 2:30:42 GMT -5 by herwinw: Added blog post