I've only played in one 36-hole qualifier, for the US Am, at Colonial CC in July or August. I was in my early 30s, a serious jogger, and in very good physical condition. The conditions were hot, humid, and whatever little wind there was, it was shielded by Colonial's majestic oaks and pecans. The bent greens were baking as we played- some of the shots into them plugged; the fringes were mostly burned out and played as GUR.
My caddie doesn't show up for the morning round, but I got around in 80 and didn't feel bad despite being out there over five hours. My guy shows up for the afternoon round, but due to the slow pace of play, I barely had enough time to change underwear and grab a hot dog. What little recollection I have of that hazy afternoon round is that I shot around 90, well back of the number, but not high enough to get the dreaded USGA letter. His recollection is that I hit the ball nearly as well as one of the guys in our group who did qualify- a college player from El Paso, but that I just couldn't get the ball in the hole. Whatever the experience or the truth might have been, I never tried the US Am qualifier again.
I did carry 36 holes for four straight days at the Bandon KP this year, and, for the most part, my body held up pretty well. Cold and rain aren't much fun to play in, but it beats hot and humid any day.