IME the psychological ability to race long distances and suffer really stemmed from how much importance I attached to an event. I have raced Ironmans where I was not in my best shape, however achieved a better performance (Austria 2005) than when in good shape (Austria 2004), because I had simply attached more importance to the race and was willing to hurt myself more for it!
However, having said that, the old addage "Train hard - win easy" really does have some truth to it. The suffering, intelligently calculated suffering that is, should be reserved for training. So that a race, with a well recovered and rested body, just feels easy (ish!). With the right preparation and when in peak condition, even an Ironman can be pushed and feel relatively easy.
I have heard European IM winners being interviewed stating that they just couldnt hurt themselves during the race, saying "It was like a dream - effortless speed!"
But, back to the original question...
When designing a training programme, I like to look at specifically what is required physiologically in the event being targeted, then work backwards from there. A common phsiological theme among elite endurance athletes, especially over longer race distances (2hrs+), is muscle fiber composition. Having a high % of type 1 slow-twitch fibers, and well trained type2a fibers being of the upmost importance. Categorising fibers into 3 or 4 sub-types is really a reflection of the means by which phsiologists measure, detect, and label the fibers. The reality is probably better described as a sweeping continuum of fibers, ranging in characteristic from highly unfatigable ST to stronger but easily fatiguable FT. There are many arguments and counter arguments surrounding the plasticity of muscle fibers, that is their ability to change their charateristics from one type into another... some so much so that they even display altered gene-expression. (This probably only applies in one direction though, FT towards ST, with FT gene expression being the default gene-expression) Personally I like this argument as it puts us back in control. Rather than being victims of our genetics, we may have the ability, at least in this small area to change things providing we are prepared to put in the measured effort.
So, my first step in designing the optimal aerobic training protocol would be to target muscle fiber endurance charateristics through as wide a range within the fiber continuum as possible.