Admittedly my mental strength has come from experience but the whole ultra thing is about the physical ability to complete and mental strength.
Pacing and nutrition become mental issues when you have to recover and continue from either mid race. You are spot on Daz, people do just crack. The option to pack it in is the easy way out. If I'm feeling low and susceptible to dropping I will enter an aid station with what I'm going to do mapped out in my head. Then I go in eat, drink, lube etc and then move on, no time to mull over dropping.
Blisters I can relate to, I have had some monsters in my time, never dropped because of them (yet) but they do come under the injury banner. People wearing new shoes to an ultra never fails to astonish me.
DNF'd my first 100 and since then I haven't entered anything I didn't feel I could physically finish. I can't be certain i'll finish but I'm pretty damn sure my body is capable of doing it.
Every year my A race becomes increasingly harder and sooner or later i'll be caught out but it will only be through injury, illness or being timed out. Tiredness is not a reason to DNF. Next years A race will be no exception!
So in answer to the question Yes I do expect to finish because that is where I need to be mentally. Of course I may not but I expect to. The same surely must be said for a beginner doing there first 50. If you go in mentally right, why wouldn't you finish? If you only drop because of illness, injury or being timed out you can only succeed.