2d 19h 59m 32s
I completed the first known non-stop circumnavigation of London's M25 motorway in May 2017. This was my second attempt after taking 80+ hours in 2016 with a 36 hour break in the middle. This time I completed the route in a shade under 44 hours with the added achievement of doing it without sitting down from the time I started at 8am on the Friday morning until I finished at 4am on the Sunday morning.
I had some support along the way - the first and last 6-8 hours plus a friend feeding me breakfast on the Saturday morning. The rest of the time I purchased food from shops along the way.
My full write-up about this FKT is on my blog: http://richardwalkslondon.com/m25-motorway-circumnavigation/