Hike Day: Precautions
You will need to drink plenty of water or sports drink at all times to remain hydrated,and eat plenty of salty foods or snacks, especially later in the hike. Salts are very important in order to maintain electrolytes and replace calories. Such salty foods will be available at the support stations.
It is dangerous to drink too much water without salts, so you need to do both. Please don't forget to eat - even if you're not hungry, a light snack could do your body a lot of good.
Note that there are water pumps at on the towpath at hiker/biker campsites. The National Park Service checks their water once every week or two to ensure that it is potable; if it is not, they lock that pump.
Note: You should not rely on the water pumps to be available; plus they require significant effort to prime.