A folk rule with no pedigree
The "eight 8-oz glasses" standard has no clear scientific origin — it likely descends from a 1945 recommendation that also mentioned most of it comes from food, a clause that got lost in repetition. Actual adequate intakes (National Academies) run 91–125 oz of TOTAL fluids including food and all beverages — bigger, and measured differently, than the folk rule.
What actually regulates hydration
Thirst, evolved over millions of years, is remarkably accurate for healthy people in ordinary conditions — it triggers drinking at ~1–2% deficits, right where performance begins to care. Urine color and morning weight add objective checks. The rules of thumb are targets for planning; the body's own gauges are the daily instrument.
The overhydration footnote
Hydration culture occasionally overshoots into forcing water past thirst — harmless at the margins, dangerous in long events where exercise-associated hyponatremia has caused real deaths. Drink to the plan AND the signals; more is not automatically better, and your kidneys have opinions.