Lack of water = dehydration
Most triathletes and marathoners don’t typically go into severe dehydration, buteven moderate dehydration can wreak havoc on your pacing. Prevent dehydration by knowing your sweat rate. Knowing your sweat rate will help you determine how much you should drink.
Here is a quick and easy calculator you can use:
Experiment with what you discover. See if it works for you. My recommendation would be to use water to rehydrate rather than a sports drink because the added sugar in the sports drink can cause stomach issues. Make sure your hydration is part of your nutrition plan. Don’t forget the electrolytes as well.