I've quickly read through the first 151 posts of this thread, and have yet to see a correct statement of the law regarding waivers.
Although the interpretation of the waiver's effect is ultimately a question of state law (here, I presume that the substantive state law would be that of Minnesota, which I haven't researched), waivers typically provide protection against claims of ordinary negligence, but are unlikely to protect against claims of gross negligence. I don't recall ever seeing any "nefariousness" requirement for establishing gross negligence. The problem, though, is that the distinction between ordinary and gross negligence is a pretty fine line. Although I have seen a court determine the absence of gross negligence as a matter of law rather than a matter for the fact-finder (which would likely be a jury), I wouldn't want to be on the defense side of that issue if two or three people die in a marathon. Also, one could argue that putting on a marathon under these conditions is "inherently dangerous" conduct, which might also avoid the defense of waiver. (Not that I would be the least bit inclined, as a judge or juror, to find for the plaintiff in these kinds of cases.) I also want to note that, in marathons with a large number of, let's say, "recreational runners," the likelihood of death from dehydration or some kind of overheating may well be less than the likelihood of death from overhydration resulting in abnormally low concentrations of sodium (hyponatremia). I'm not sure how a marathon organizer could reasonably be found to be grossly negligent for the death of a runner who overhydrates himself to death, but I'm confident that I could find a lawyer and a well-paid "expert witness" who would claim that offering water without electrolytes added is grossly negligent conduct in light of the known dangers.
That said, it's a shame that concerns like these need to be considered under such circumstances. I personally wouldn't want to be a race organizer with any possibility of liability for injury, illness, or death. Also, it's a shame that the Twin Cities Marathon, which I ran a couple of times in the '80s and was widely considered the best marathon in the country, is now getting cancelled for something like this. (I realize that some will say that the cancellation is justified because of the medical risks to the participants, not the legal risks of the race organizers. I'm pretty sure that I would not find that persuasive.)
Now for the usual caveats: I'm not providing legal (or medical) advice to be relied upon in any way. Consult your own lawyer (or doctor) for that. These are just the musings of a lawyer who apparently has too much time on his hands.