Most runners guess at water. A short weigh-in on a run that feels like race weather is a better plan than a new bottle on the start line.
The test
Weigh yourself in light clothes before a 20 to 30 minute run. Do not drink during it. Weigh again in the same clothes after, and towel off first. The drop in pounds, times about 16 ounces, is the fluid you lost. Divide by the hours you ran. That is ounces per hour for those conditions.
Race day
Replace most of that loss, not all of it, across the hour. A hot race raises the number. A cool race lowers it. Practice the same drink on a long run. The marathon nutrition guide covers carbs. This page is only the fluid.