It appears xfit dude has never been in the military when he talks about SKINNY/SCRAWNY runners.
Back in the day I went to MCRD San Diego for a few months of fun. At the beginning of boot camp, during one of our first hygiene inspections, the drill instructor when around with specific instruction/orders on a variety of "deficiencies" to specific people. About a half dozen guys were put on half rations, some how to cut their nails properly, some to shave completely and a few other things. I was third to last to get inspected. When the DI asked why I was so "thin" I said "the private is a runner". He next asked me, "what do you think about going on double rations to gain weight". Not wanting any special treatment, I thought quick and responded, "the private has a high metabolism and won't gain any weight". This brought chuckles from the crowd until the DI said shut up. I ended up on double rations and gained all of 3 pounds or so during boot camp. I'm 6'1" so I went from 152 to 155, still skinny. I was a 17 low 5k xc guy, and during our pft's I was 18 low 3 miles. All situps and 14-15 NON kipping pullups in the other pft events. In our series my run got me 2-3 and overall pft score higher than most.
Since xfit guy is likely not familiar, we did a fair amount (though less than xc and track) of running, marching and hiking, these were done to help build/maintain endurance. As the skinniest dude I kicked everybody's ass, except for one, in the STRENGTH AND ENDURANCE course, even the DI (couldn't pass him because he said so). I was jogging so at the different stations the strength disiplines didn't take much out of me. Mount M***********r was easy as was the little hike on the other side of San Onefre. What's the take away from this for xfit guy and his ilk.
Us runners have endurance because we run and we like it. By having this endurance it lets us do everything else well, functional and otherwise.