The latest guidelines suggests that runners should aim to drink 400 ml to 800 ml per hour maximum, the higher rates (800 ml) for faster, heavier runners competing in warm or humid conditions and the lower (400 ml) per hour for the slower runners competing in cooler conditions and those expecting times of 4 hours or longer in a Marathon, or 2.5 to 3 hours for a half Marathon.
“There is a real risk of slower runners becoming overhydrated and developing dangerously low blood sodium levels if they drink more than 800 ml per hour.”
Dr. Terry Cooper (Adviser to “Shakespeare Marathon”)
Healing Foods 