Hydrating beforehand is essential. I learned that in the Marine Corps. Its far easier to maintain hydration if you are juiced up beforehand then it is to rehydrate when you are dehydrated. I drink big time fluids at night in preparation for the next day during the warm season.
Even my bird dogs, I mix their dry food with water when I feed them. They slug the water as they are eating their food. I can guarantee that they are properly hydrated according to their body weight on any given day. But before we hunt, and I know one of them is going to do a hard 10 miles, I add more water to their food. Molly, my german shorthair, wont stop for a drink while she is hunting, which doesn't bother me because every 15 minutes she is stopping to take a leak. So prehydrating her obviously works.