Mark Sisson’s entire idea is based off of the idea that long distance athletes train at what’s called tempo pace. Tempo pace is about 80-85% of maximum heart rate.
He went on to argue that consistently training at long distances increases cortisol levels and said this is bad.
There are two problems with this. First, distance runners don’t do all of their training at tempo pace because that’s stupid. Second, cortisol isn’t all bad - the body needs to be over stressed at points in order to create the adaptations that we are exercising for.
Point being, you’ve got to be careful just like you are with any form of exercise. If you destroy your body day after day for long enough, bad things will eventually happen to you.
The important thing to always remember is that more runners die from drinking too much water than die from not drinking enough. Everything can and will kill if taken to the very extreme.
