The Chilterns are the closest range of rolling hills to London that provide a getaway from the city (you can get a train to Princes Risborough where the route starts indeed from the city that makes it accessible to many budding FKT’ers!)
The Chilterns Hills are a beautiful part of the UK countryside - designated as an area of natural beauty - that are steeped in history (the famous Ridgeway Trail runs through this area), and offer some amazing trail running.
The Chilterns 3 Peaks Challenge in its current form is actually a charity walking event that takes in the three peaks - Whiteleaf Hill (244m), Coombe Hill (260m), and Ivinghoe Beacon (233m), which are iconic hills in the area that offer spectacular views of the surrounding area.
A link to the current challenge can be found here: https://www.facebook.com/chilterns3peakschallenge/?locale=en_GB and on the UK National Trail website: https://www.nationaltrail.co.uk/en_GB/event/chilterns-3-peaks/
Inspired by this event as an FKT I have based this on an out-and-back route that makes this an ultra distance of 57KM / 1006M - a distance that is tough but achievable in one day that make it accessible to hopefully several attempts. The route is predominantly trail and off-road, with some sections on pavement passing through towns.