Route: Ramsay Round (UK)
"The Ramsay Round is a long distance fell running challenge near Fort William, Scotland. The route is a circuit of 58 miles, taking in 24 summits with a total climb of around 28,500 feet. Ben Nevis, Great Britain's highest peak, is included in the route along with 22 other Munros [summits]. Originally, all 24 summits on the Ramsay Round were Munros, but Sgorr an Iubhair was declassified as a Munro in 1997. The route was devised by Charlie Ramsay as an extension to an existing 24-hour walking route [the Tranter Round, see below], and first completed by Ramsay on 9 July 1978.
Charlie's completion created Scotland's Classic Mountain Marathon. The aim is for participants to complete the route, on foot, within 24 hours. Runners must start and finish at the Glen Nevis Youth Hostel, and may run the route in either a clockwise or anticlockwise direction."
More information and history is on the Ramsay Round Webpage. The site records 96 sub-24-hour finishers through 2016.
Tranter Round: A related challenge is the Tranter Round, which covers a sub-set of 20 summits over 63km with 6100m of ascent. It was first completed in June 1964 by Philip Tranter (see this page for some history).