Just want to point out that Chris' version of sedentary is a far cry from what most might quantify as such. He really can't sit still for long. ;p
I had the same issue a few years ago, in fact I couldn't stop losing weight even though I ate 4 or 5 huge meals a day, everyone told me I needed to gain weight, including my doctor, but I felt I was in the best shape of my life.
Chris's metabolism is definitely jacked, but regardless of whatever quantification a physician would put to it, it's a natural reaction to the sheer amount of activity he sustains on average. When I was working outside full time, exercising, and participating in strenuous sports, it was the same. I don't think your body can run full-tilt 90% of the time and then just throttle back.
In fact, even after my lifestyle became much slower, with orders of magnitude less exertion, my metabolism stayed elevated, for well over a year. Only now after continued loafing have I managed to reach a relatively normal weight. Which btw, feels fat to me, but according to my doctors is in line with where I should be. Regardless, I know I'd be feeling healthier 5 lbs of beer lighter. ;)
Ultimately, what I think Chris is really getting at here, and starting to really understanding himself, is that there's definitely a line you can cross. Just as it can be hard to stay healthy in an overly sedentary lifestyle, it can be with an overly active one also.
The important thing is that he's acknowledging that, where as many people I see that become obsessed with activity and fitness, never realized when they've crossed that barrier. I don't think our culture really allows us to believe that you can be too "skinny" unless you've got an eating disorder, but you can, and as Chris is seeing, it can have consequences. It's especially compounded by the fact that's he's not "frail" at all, he's very muscular, however, most people with his physique get it from a gym, and only put it to test in a gym.
Peak physical "strength"(endurance, fitness, whatever) and physique are not synonymous, even if they often line up.
Don't worry Chris, if we keep with the Tequila and post hike Waffle House diet, it's bound to take it's toll eventually, even if we do 32miles in 12 hours.. =\