An above average runner doing a three-hour marathon could expect to lose 3-5 kilograms of weight. For every kilogram you lose, you need to drink 1.5 litres of water to replace it.
And it's not just water. Sweat also contains salt (mainly sodium and potassium) which helps the body absorb water. Over a long race like the marathon, you can sometimes see (or taste) a fine layer of salt forming on the skin.
The best way to gauge how much fluid you've lost is to weigh yourself before and after a race - preferably naked, or with sweat soaked clothing removed.