The Bob Graham Round is a fell running challenge in the English Lake District which involves traversing 42 fells (hills) over a 66-mile course with 27,000 feet of elevation gain. It was first done under 24 hours by Bob Graham in June 1932. An related challenge is to summit the most fells in 24 hours. The Bob Graham 24 Hour Club keeps track of people who have repeated the sub-24 hour, 42-peak traverse, and numerous area records. Besides the Round of 42 summits, records are also kept for the most summits in 24 hours, as well as for the rare Double Bob Graham.
Note that by tradition the Bob Graham Round is supposed to be done with supporters and pacers.
Salomon put out this excellent video about the Bob Graham Round: https://www.youtube.com/watch?v=f-03GxweU30