First, let me say I have a great respect for letter carriers, and other people who do jobs with a big physical component like that. I know I wouldn't last an hour in such jobs! And I can see how it'd be important to stay hydrated in such a job.
But when you say you struggle to drink 64oz a day, why is that? How do you know that's the right amount of water to drink? And forgive me if I'm thinking too simplistically, but if I had a goal to drink 8 cups of liquid a day, which is what Google tells me is 64oz, I'd simply set up a schedule of drinking a cup 8 times a day, at pre-set intervals. Like, 1 cup when I wake up, another cup with breakfast, 1 cup around 10, 1 cup with lunch, 1 cup around 3, another around 5, 1 cup with dinner, and 1 before bed. This might be simpler for me, because I work in office or from home, so I can just get a drink any time I want. Is your difficulty that you are working outside, delivering letters, so it's hard to take regular drink breaks? Or are there other reasons?