<i>"... But none of that wisdom has prepared me for this. Eric warned me that running low on water during a 12-hour run in 85° heat would be crucial, and now, with my pee the color of convenience-store coffee, I know I'm too dehydrated to finish. ..."</i><p>Not very bright. Dehydration means approx. 10% loss of performance and potential for damage. Isotonics work well in this situation. But 12 hours of water. You cant carry it. It has to be a refill about every 2Hrs.