Just sharing a suggestion of what has worked for me (3 marathons, successive improvements down to just sub-3). I take fluids at every water stop, often alternating between water and whatever sports drink is available. Once your body becomes dehydrated, you won't consume enough water to "catch up" before the end of your race... just as important in warm weather as in cold weather.
When I am approaching an aid station, I will usually announce what fluid I prefer: "Water!" or "Gatorade/Powerade!" It takes the guesswork out of wondering what tables have what fluid you need.
Additionally, if you're not prone to high blood pressure, I found that salting my foods leading up to a race helped tremendously with hydration. Your body will crave water to balance the extra sodium in your system. No need to go crazy with the salt, a little more than usual should be fine (adds to flavor too!).
Hope that helps, good luck!
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