I never doubt that I can do the distance. Even in the very long stuff that I do, I always have absolute faith that my body (barring injury) will go where I tell it to go. The issue becomes whether my head wants to tell it to go further.
I'm absolutely convinced that the longer the distance the more it becomes an almost totally mental thing. Unless i am injured, my legs will always take another step, the pain will not be there forever and I can finish - but the mind has to want to, and allow you, to keep going.
I know that if I get my head in the right place then I can do any race. I have been in 100's+ where I KNOW, with an unshakable belief, I am going to finish after 10 miles . I also have been in races where my head is in totally the wrong place, I haven't 'wanted' it enough, and a DNF is inevitable.
Interesting, I am increasingly finding that the less mental energy I use pre-race the better I run. Some of my best runs have been when I literally just turn up with minimal pre-thought. A minimal mental taper.
Always learning about my body and particular my head.......which is a complex place
I have done 40 miles on the flat in 5:30. The first 43 miles of the Spine have taken me 14:30, 12:30, 13:30 respectively in the last 3 yrs. And that's near the front. In 2012, it took us nearly 44hrs for the first 108 miles and we were second in. Distance, terrain, weather, time of year, 15hr of dark, difficult navigation, full winter kit etc
Most events are against yourself not the clock....but The Spine is both. Try it......