Definitely in the FWIW category.
Back in the late 70's I ran my first marathon and (surprisingly) qualified for the Boston Marathon. A group of four of us decided to make the trip the next spring. This was the year that Rosie Ruiz "won" the ladies race (she didn't pass me, BTW).
Anyway I could never learn to drink water while running (not a problem on a bike for me, BTW). A buddy (better than me) talked me into running with him and, unfortunately it was 'perfect' weather (low 80's). Of course all my training was cold weather stuff and I had no heat training whatsoever in April.
I absolutely collapsed the last 5 miles and assumed that it was just a somewhat too fast pace. While I finished with a sub 7:00 min/mile pace, my pace the last half dozen miles was more like 9:00.
Then I returned the next fall to the Nov. marathon in Louisville, Ky. Again it was the same 'perfect' weather and now I was in very good condition. But the result was the same - complete collapse the last 5'ish miles. But this time for no particular reason I had weighed myself before the race. I could easily reconstruct all the fluids that I had consumed after the race (I just couldn't drink when running). Doing the math I calculated that I had lost 13 pounds of water during this race.
Trust me here - you don't want to do this. Twice I lost around 13 pounds in 3 hours and was never uncomfortably hot. But around 2.5 hours in - things were REALLY tough. However I never felt either hot or thirsty.
dave