Just as you need an organized plan for your run training, you need an organized plan for hydration during your training and during a run. If your hydration plan is the "I'll wait until I'm thirsty" approach, you are planning to fail. You would not think of putting diesel fuel into your gasoline motor car, or expecting your care will run on fumes! A hydration plan is as important as a physical training plan. Here is an article with good basic information on hydration from Run the Planet.