Via Dave I found Dicky's comments about pushing yourself too far. There's a great quote in the comments attributed to Eatough:
I just think it came down to who was willing to destroy themselves the farthest, and Craig was more willing to go further. I also think that's why he's [in the hospital] and I'm [at home]
I've often thought of pushing my limits as one of my limiting factors. Once you hit the expert level and up in mountain biking, it's not uncommon to see guys come across the line and literally fall off the bike once they stop pedaling. They've pushed their body right to the brink. I heard an official at the very first road race I ever did (a 5k TT) comment, "if [the racers] time this right they should vomit about 20 feet past the line". I've yet to push myself that hard.
I have found that "extra" energy out on the course and been able to pick the pace up when I thought I was dead. The last two laps at Nationals this past year, it hit me where I was at and that I'd been saving up some energy that was about to be wasted so I pushed the pace and picked up a handful of minutes on those last laps. At the Dusk 'til Dawn in Gallup, I started catching glimpses of things out of the corner of my eyes and thought I was going to stop to vomit a few times, but pushed through it by convincing myself that the fatigue I was feeling was all psychological, but those are exceptions to the rule.
I normally try to stay in a state of near effortless that Dave is talking about. I've heard it called "flow". Body, Mind, and Sport is an excellent book on the topic and actually the book that formed my opinion about pushing yourself. It talks about reaching the state of flow on demand by the way you train and live.
For me, that flow is right below my ventilatory threshold where I'm pushing at that point just below breaking. It's where if I keep hydrated and fueled through the day I can go all day. I often feel detached from what I'm doing. I'm aware and fully engrossed in what I'm doing, but my mind goes on auto-pilot and becomes an observer of what's going on. Instead of "rock, go left", my mind is commentating "rock, I'll miss it to the left". That feeling, that effortless, that dissolving of the mind as part of the body and the body as part of the mind is as close to nirvana as I think I'll ever come.