Right, I admit that I was completely wrong when I said that only 10-20 men had done it, so here's an updated list of men and women who have definitely (or have a good claim to have) run 300 miles in 3 days:
Modern era (from DUV): James Zarei, Yiannis Kouros, Ramon Zabalo, Jean Boussiquet, Wolfgang Schwerk, Gilbert Mainix, Joe Fejes, Stu Mittleman, Bryan Smith, Sigi Bauer, Serge Abona, Hans Heise, Bob Hearn, Eoin Keith, John Geesler and Jakelaitis Rimantas.
Pending Ratification (but definitely did it): Matthieu Bonne, Stine Rex, Camille Herron (According to Wikipedia)
Possibly but not certain: (ie over 575 miles in 6 days) Joe Record, Dan Lawson, Olivier Chaigne, Claude Hardel, Jaroslav Kocurek, Tom O' Reilly, Kevin Mansell.
1880s Era: George Littlewood, Charles Rowell, James Albert (winner of 'best moustache' competition)
Westfield Run: David Standeven Dusan, Richard Tout. (How sure are we that the race distances were accurate? Some amazing performances on a road course with hills and undulations which were well in advance of what they achieved on the track.)
And finally from TikTok: Nedd Brockmann!
Anyway, that makes 51 definite and 10 possibles. Feel free to nominate any more.