When hiking Whitney with Gary, Bulldog34 . . . he said he sometimes feels it more than once at the early stages of a hike. Once again, it goes away for him (If I remember correctly.) And, it's always mental, not physical.
It's definitely mental for me Joe, but it's physically based - it's usually the result of the first lactic acid buildup in my legs. Whenever I get that that first hard burn, I instinctively want to stop and turn around. I never do, of course, but it's something I always have to fight through early on in a hard uphill hike. Once I've achieved an oxygen and acid balance I'll generally keep going till the cows come home.
My quitter tendencies here are the product of running the quarter-mile (440 yards/400 meters) in high school and college. That is the most intensely painful race in the track world because you're asking your body to run at 85% max speed for about 200 yards more than your legs think is reasonable. Lactic acid buildup is enormous the last half of this race (along with an intense oxygen debt) and it hurts like hell. Whenever I get that burn in my calves or quads, I flash back to all those 440s I ran - and all the pain that came with it - and I just want to say "screw it!"
I really hated the 440 due to all that misery. I much preferred the 100 and 220, but by the time I got to college I wasn't quite fast enough to play with the big boys in the pure speed events and I didn't have the endurance for the mid-distance 880. So I got pigeon-holed into the quarter-mile and hated every single race I ran, regardless of where I finished. In fact, the last race I ran before I hung up the spikes for good was against Edwin Moses in an open 400. He already owned a couple of Olympic golds at the time, but I was actually leading at 300 meters. In reality Edwin was just cruising, and about 80 meters later he was several steps ahead of me and pulling away. I decided then and there that I was wasting my time, and that it was time to find something I actually enjoyed doing.
Hiking is much more my speed - once I get past that initial burn . . .
