I've recently been reading a great post on Stuart Mills' blog which is about self expectations and fatigue.
I've been a runner since 1972 and in that time I reckon I've learned quite a lot about myself - most of it since 2005 when I decided to try ultra marathons.
For me, self expectation is derived from training. Over the years I've found that generally my mental strength is very good and that I'm usually good at sticking to any race plan I may have. Of course, having a race plan in the first place entails estimating my fitness at the time of the race. This estimation is largely built upon knowledge of previous races and comparing the training for them with recent training. If I get this estimate wrong one of two things will happen:
(1) I'll finish the race knowing I could have done better, or
(2) fatigue will develop which will result in the inevitable reduction in speed.
In the above paragraph 'fitness' is not the same as the dictionary definition: "capability of the body of distributing inhaled oxygen to muscle tissue during increased physical effort."
I have found that there is definitely a mental aspect and in ultra running terms I would suggest that a better definition of 'fitness' would be: "preparedness to compete to the best of one's ability." Therefore 'fully fit' would mean 'fully prepared'.
Being prepared naturally involves training the body to meet the physical demands of the race but it also means being aware of, and planning/training for, the other issues that arise in very long races. This is the mental aspect of fitness. If I'm confident that my training has gone well AND my race plan is sound then I know I'll do well. In my last ultra I found that the icing on the cake was having someone unexpectedly tell me before the race that I was capable of winning.
For a long ultra my race plan will involve a knowledge of nutrition, hydration, local weather conditions, course layout (and surface, etc).
The mental aspect also comes into play when one or more aspect of my preparation has been at fault ... of course, I usually don't realise that my preparation was wrong until the last third of the race.
Tim Noakes also provides an insight into fatigue and his definition focuses almost exclusively on the mental aspect (which he calls the Central Governor Theory):
"... the increasing feeling of fatigue and the progressive reduction in the capacity of the exercising muscles to maintain a constant work output during prolonged exercise results from currently unrecognized processes in the brain, which presumably act to prevent bodily harm during such exercise. This model theorizes that performance during exercise is determined by two separate phenomena:
(a) A pacing strategy that is preprogrammed into the athlete's subconscious brain as a result of previous training and racing experiences.
(b) Acute alterations to that preprogrammed strategy resulting from sensory input from a variety of organs - heart, muscle, brain, blood and lungs, among others - to the exercise controller or governor in the brain. Output from the controller to the motor cortex then determines the mass of skeletal muscle that can be activated and for how long, thereby determining the pacing strategy that the subconscious brain adopts during exercise.
At the same time, information is sent from the controller to the emotional and other centers in the brain. These influence the level of discomfort that is felt, the emotional response, and the self-talk and self-doubt that are additional but poorly understood features of the fatigue that develops during exercise."
Understanding fatigue and how to avoid it and/or cope with it is one of the keys to successful ultra distance racing.