What a long run really costs you
Running heats you up quickly, and sweating is how your body gets rid of that heat. Most runners lose somewhere between 0.5 and 2 litres an hour, and where you land depends on your pace, your size, the weather and how used to the heat you are.
The problem is that you rarely drink as fast as you sweat. Once the deficit passes roughly 2% of body weight, research consistently shows heart rate drifting up, perceived effort rising and endurance capacity falling, and heat makes all of it worse.
For a 70kg runner, that is 1.4kg. Utterly normal on a muggy 90 minute long run.
The fade in the final mile usually started an hour earlier, in fluid you never put back.
Work out your own sweat rate
There is no universal bottle-an-hour rule, but there is a simple test. Weigh yourself before a run and again straight after, towelled off, and note whatever you drank on the way. Every kilogram lost is roughly a litre of sweat.
Do it on an easy run and a hard one, on a cold morning and a warm one, and you will see how much the number moves. Most runners are surprised by how much a mild British summer changes it.
Between sessions, the quickest daily check is your first trip to the loo. Pale straw means you are generally topped up. Dark, like apple juice, means you started the day already behind. (Vitamin supplements can turn urine bright yellow, so do not read too much into one reading.)
Two weigh-ins tell you more about your hydration needs than any generic guideline ever will.
Why water alone will not get you round
Sweat is not just water. It takes sodium with it, and over a long run that adds up. Plain water puts the fluid back but not the sodium, which is why it can feel like it goes straight through you.
Sodium is the key electrolyte in hydration. It supports fluid absorption in the gut, encourages you to keep drinking and reduces how much you lose through urine. Carbohydrate matters too: on runs past about an hour it gives working muscles fuel alongside the fluid.
So for anything long, warm or hard, drink something with sodium in it rather than water alone. For a half hour plod round the park, water is fine.
Water gets fluid in. Sodium is what keeps it there long enough to run on.