The longest distance i ever managed in a 24hr period, was 83miles, whilst attempting my first of three LDWA Hundreds, back in the 1980s.
There was a position i reached, that no amount of encouragement from fellow competitors, could make my tired limbs and body, continue the last 17 or so miles to the finish line.
Those of us who have been in a physical condition, that ones entire body is so drained and physically incapable of continuing to move ones legs, will know what i mean.
Its been some time since ive walked a real great distance, but i recon somewhere between 25 to 35miles would be my limit nowdays, but if the terrain was really challenging, then who knows.
I am considerably older, and not as mountain fit as when i was in my late twenties early thirties.