After years of hiking with no problems including frequent death marches in NH, I hit the wall a few years ago on a hike on the island of Dominica. Billed as the best hike in the Caribbean, the 8 mile round trip was to go through the rainforest and up and down a small 3,500 ft mountain to see The Valley of Desolation (a place with volcanic vents, multi-colored hot streams, and sulfur crystals on the ground) and then The Boiling Lake (which is what it sounds like, the largest one in the world). Total vertical (up and down) was only around 5,000 feet.
The hike is infamous for beating people up - wet, muddy, steep, slippery, etc. After only a couple of hours, I knew it was going to be a huge struggle for me, and the worst part was that I had never experienced this before and had never slowed a group down to wait for me. I was dying for a good 8 hours. I presumed it was dehydration but really have no good explanation. I do know that when we got back to the first decent-sized river I threw myself in it and drank and drank (there is, reportedly, no problem doing this on Dominica).
The point is that even the most experienced among us can without warning encounter a situation for which in some cases the only solution is to struggle through it as well as possible. Weather conditions, terrain, etc., had little to do with how I felt that day, and yet I don't know that I would have done anything differently feling as good as I did at the beginning. I did get a lesson in humility, though, which has certainly stayed with me.