Found this on the South West Coastal Path. Geo-caching running (as long as you can find them!) where the SWCP has the shortest ultra distance for then "extreme" version. This is commensurate with the runs that I have had on the route compared to the SDW and the Ridgeway and NDW. It is a knarrly hardcore run........I will do the 630 version but unfortunately it only goes up to 85km!