The Power of Four Ski Mountaineering Race in Aspen, Colorado a couple of weeks ago was an adventure into the physiologic unknown for me. Sure, I've done plenty of exercise of that duration and longer. I've also gone just as hard for periods of time. But I've never really combined that kind of duration with that kind of focused effort. There just aren't competitive events of that length happening that often. I think endurance mountain bike racing will be the venue for similar efforts as that sport continues its growth but since I don't own a mountain bike right now, those races pass me by.
Qualities that matter
Although I love the shorter, more intense skimo events commonly held these days, longer races probably suit my aging physiology more. These events do not necessarily favor pure, high-end aerobic power but rather sustained, sub-maximal output. Certainly, it stands to reason that someone with a VO2 max of 80 ml/kg/min working at a sub max rate is going to going faster at 80% than the guy running at 80% of 65 ml/kg/min. However, energy substrate utilization comes into play here and this is harder to quantify. Nevertheless, this factor, along with the slippery slope of fuel and hydration management are the two determinants in a race like the Power of Four.
With everyone skiing an even race technically (downhill, skinning technique, transitions), the finishing order in many races can be guessed based solely on aerobic power. As a graduate student in exercise physiology, I published a study involving elite women road cyclists, correlating their finishing times to VO2 max in a multi-day stage race. Needless to say, the correlation was high. My conclusion was that pure aerobic power was necessary for success in the sport and predicted finish placing more often than not. Just as race tactics can cause some variation in these correlations in road cycling, technical factors can play a significant role in the outcomes of shorter skimo races.
In longer ski mountaineering races involving less transitions and more straightforward descents, technique becomes less decisive than the physiologic qualities listed above. And this, apparently, was my ace card at Aspen. Couple this with some solid team work with my partner Jared where we towed each other with a light bungee cord when we each had a bad patch and we ended up better than all but one team.
Aspen Power of Four specifics
Using my Suunto T6 computer on a regular basis has allowed me to monitor performances in my races this year. Looking back at my first race this year in Jackson Hole, the "fly and slowly die" pattern was evident. I went out hot, too hot, and faded as revealed in the downward slope of my heart rate over the course of the event. Some of this was due to conditioning, pacing, energy substrate oxidation and fuel intake. With each passing event, repeated analysis has inspired beneficial adjustments to these factors creating better performances. With all the variables at play in skimo events these improved performances are not always represented by higher placings so I don't necessarily look there for validation.
The Power of Four event is somewhat of an outlier due to it's length but the graph is still tell-tale. 
Draining my glycogen stores was inevitable and early carbohydrate feeding was essential to stave off this problem as long as possible. Anticipating about 5 to 5.5 hours of effort, I brought about 1,500 kcals of food consisting of Gu energy gel and Gu Brew energy/electrolyte drink. The race ended up being about 20% longer making adequate calorie intake unlikely. I knew that proper pacing would optimize substrate oxidation toward more fatty acid metabolism. I think this is the only reason I did not bonk when we put the screws to the team ahead of us and moved into second place. Going just a little harder at any number of places earlier in the race could have ended that run at glory by tapping reserves I needed at the end.
During the last hour of the race my HR dropped below 150 bpm for a short time and I feared that the end was coming. However, as you can see from the final few minutes of the climb, I was able to get back up in the mid 150's for the last push indicating there was still something left in the tank. Next year I will have another 300 kcals on hand for the final 30-40 minutes and should be able to charge the last part of the climb without fear. - Brian