|Uncertainty is pretty much guaranteed when you have no idea what's ahead|
In my opinion (now) there are two separate confidences that need to be considered. One can be thought of as a confidence in one's physical body or physical abilities. The other is more correctly considered a confidence in the ability to deal with the unknown. My previous treatise on confidence really only considers the latter of these two, and it is important to recognize that while this second type of confidence can be an asset to the aspiring endurance athlete, it is not required in many cases.
Thinking about these two types of confidences independently explains a lot. It explains why some very gifted ultra endurance athletes that have the former ('physical' confidence) often don't excel at endurance events that require a confidence with the unknown - such as expedition length adventure races.
It also explains why traditional training methods (high volume) are so successful in preparing people for ultra endurance efforts - high volume may provide this first type of confidence. For people training for typical Ultra Endurance Events such as an IM or traditional ultra-marathon, doing LOTS of work provides some exposure to the type of physical and mental demands that are going to be present on race day can go along way towards providing the peace of mind that they can meet these challenges. Furthermore, adherence to a program that claims that it will prepare you for a certain effort is certainly going to go some distance towards making you feel psychologically prepared - simply through the placebic effect. Of course traditional actually does get you fit - but so might ultra low volume training. One difference is that there isn't a big body (or any body for that matter) of evidence - anecdotal or otherwise - suggesting that low volume does prepare you. So if one is seeking a way to get fit and develop that physical confidence that will help ensure race day success for most conventional ultra distance efforts (and this is the boat that many people looking to get into ultra distance racing will find themselves in), then traditional methods of training clearly are the only viable route.
On the other hand, there are some individuals that already possess confidence in their physical abilities. For this group, i continue to contend that low volume training, high intensity training would offer a route to a level of fitness that would be suitable for success at mainstream ultra-endurance events such as IM.
Confidence in the face of the unknown, however, is a bit harder to nail down. I don't know of any systematic way to develop it - other than than trial by fire - putting yourself in difficult situations rife with uncertainty and seeing what happens. If you get through these situations, it seems natural to assume that you will develop this confidence over time. Many/most people may never aspire to this type of confidence, although i personally feel it undoubtedly proves beneficial to those that attain it. And of course anyone attempting the type of ultra endurance event that is based on the unknowns - expedition adventure racing - is more likely than not to fail if they don't already possess this brand of confidence. Interestingly, in these events this confidence is far more important than top end fitness - athletes who excel at more 'controlled' ultra events often are no match for athletes with mediocre fitness that revel in uncertainty. Because of this, the low volume approach will often work well for these 'mediocre' athletes as well.