Also drink throughout your exercise. You need liquid as fuel.
(Don't drink a gallon just because some blogger said.)
Dont force it and waterlog yourself, just drink when your thirsty.
Once you are fully hydrated, there will be no reason to "drink water before you excercise" You will already be hydrated.
A rule of thumb if you're regularly dehydrated is; drink until you feel like you'd puke if you drank more. Take a break. See if you can drink more than that. Then when you recover, work out (Drinking 50 fluid ounces was a lot and under pretty intense conditions; she made it a rule for me after I passed out from heat stroke twice. DO NOT drink more than 80 fluid ounces in an hour).