Ali's done her research! ^
They say it's hard to "over hydrate". But "pre-hydration" also helps. Drink plenty of water prior, and some "Staminade" powder mix amongst it. Say 40 & 20 mins prior. Then slug a little more before you head off. You should be good to go for a solid 45 mins without fluids.
But don't do what I do... Beer chasers at the end, sitting waiting in a cooler pouch in the car with an ice pack!