Water Consumption

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If I'm exercising in 80ºF weather, how often should I be drinking? How much?

I have a long walk tentatively planned for 1 August. If I'm walking 4mph should I be taking in about a quart per hour, more or less?

Any guidelines or resources you use would be appreciated. I have simply been hydrating heavily before going out then refilling when I return but now it's getting warmer and I will be unable to finish before rehydrating for the Ocean Walk.