Proper hydration is critical to enjoying your run, performing well, and staying safe. Since the body is more than 70 percent water, it's crucial to replace what you expend. If you're planning to go for a long run make sure to carry water with you. For really long runs you can use a water bottle or a hydration system to make sure you drink at regular intervals. Make sure you hydrate sufficiently before and soon after exercising.
Dehydration can lead to muscle cramps, heat stroke, premature fatigue, or even muscle injury.
A general guideline is that you should drink 16 ounces of liquid per hour, but it really depends on the environmental conditions and also your own physiology. If you sweat a lot or if it's very hot and humid, then you re going to need more fluids. Vary your intensity. On some days go a little bit longer and with lower exertion. On others, do more work on the hills or boost your intensity in other ways.