Once the race starts, hydration becomes important, particularly if it is a warm day. Drink, drink, drink. Do this during your long workouts in practice too. Not only will drinking fluids make your weekend long workouts more comfortable, but it also will teach you how to drink and how often to drink. Drinking while you run is not an instinctive technique; you need to practice to do it properly.
No tennis player would start a match without practicing lobs; no golfer would think a game complete without learning how to pitch from a sand trap. And no runner should enter a half marathon without figuring out how and when to drink.
Drinking while running definitely is not easy. Unless you grasp the cup carefully, you can spill half the contents on the ground. If you gulp too quickly, you can spend the next mile coughing and gasping. If you dawdle at aid stations, you can waste precious seconds. And if you gulp down a replacement drink you aren't used to, it might make you nauseous.
Drinking on the run is a science--and so you need to practice. Do this during your training runs, particularly your long training runs.
Drinking on the run is necessary for survival. Here's why. During exercise, the body usually produces more heat than you can get rid of by sweating. A marathoner's body temperature gradually rises 3 or 4 degrees to 102 degrees Fahrenheit, an efficient level for energy utilization. At this point, your air-conditioning system is in synch with the environment and you perform well. If the weather is too hot or too humid, or you become dehydrated--resulting in a drop in sweat production--the body's temperature can soar to dangerous levels. Your muscles will not perform efficiently at temperatures that are too high (over 104), so that will slow you down. This is an important defense mechanism, because if you fail to sweat and your core temperature rises past 108, you may suffer heatstroke, a potentially serious problem that can cause headaches and dizziness, and in extreme cases convulsions, unconsciousness, and death.
So drink up--but don't drink too much or too often, otherwise you'll waste time waiting to use the portable toilets along the course. You have to learn how to drink properly, and that's why you need to practice drinking during your long workouts.