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You may have heard about runners dying from hyponatremia (excessive hydration), but that doesn't mean you should go to the other extreme and not drink enough water. Even in cold weather, if you are running much more than 10-12 miles, be sure to have at least a few sips of water a few times during the run. Cold air, especially wind, is dehydrating, and you will get thirsty during a 16-20 mile run, even if it's only 20 degrees outside.
Remember that most energy gels and bars need to be eaten with some water to help the nutrients get into the bloodstream and do you some good. The amount of water you drink is individual, and as you do longer runs, try drinking 1/2 cup of water every 4-8 miles. If it's hot, you will need more, if it's cold, you'll need less, but everyone's hydration needs are different and it's important to experiment during training so you know what works for marathon day. There are formulas to calculate how much you should drink based on weighing yourself before and after workouts, but I find that listening to your body, plus some trial and error, works just as well, especially because the weather will have a significant impact on your hydration needs.