I'm actually doing it on the 20th...
My preparation...I'm doing the Marine Corps Marathon on October 31

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Haha, other than that, nothing really. For the most part you're adreneline should kick you past all the cold water stuff. Just make sure you're running a lot during the week to build up the distance endurance (its liek 12 miles or so) and then try taking cold showers at night to get your body used to the cold.