Staying hydrated during and after running is important, but so is not overhydrating. As you increase the distance (or the heat), this balancing act becomes tricker with bigger impacts on health and performance.
For the basics on running hydration (even if you’re not trail running), you can read H2-Oh, You Got This: Trail Running Hydration Basics and Trail Running Hydration: The Quick and Dirty.
It’s come to light that runners and, especially, ultrarunners were prone to overhydrating or taking in too much water relative to sodium to create the dangerous condition known as hyponatremia (too little sodium). Joe Uhan summarized this breaking realization in Waterlogged – A Dogma Shattering Book and Waterlogged Part II: Trials, Questions, and Suggestions Regarding Hydration and Ultramarathons.
For a primer specifically on exercise-associated hyponatremia (again, taking in too much water in relation to sodium), read Exercise-Associated Hyponatremia: The (Not So) Salty Truth.
In case you’re ever with someone who has overhydrated, you’ll want to check out Trail First Aid: Overhydration.
For decades, cramping was blamed on dehydration. As we’ve since learned, this isn’t the case in most running cramps, you’ll want to read up on the current thinking on cramping in Cramping My Style and Holy Cramp! The Science of Exercise Associated Cramping.
Although we always need to hydrate, running in hot weather exacerbates the need to hydrate. If you’ll be running in hot weather, check out our running in hot weather resource page.