Over a gallon of water by lunch time?

For me, that would be a LOT of water. I can certainly "feel it" when I drink too much water. I did that a few weeks ago while working in extreme heat and not eating enough (nothing at all). By around 2 pm I knew I had to eat before drinking anymore even though I was sweating buckets. As soon as I ate the "feeling" went away and I was back to drinking water again, but still, no where near as much as you've described. You're a bit bigger than me, so I can see where you'd need more than I would, but that still seems like a lot to me.

So my guess (and that's all that it is) is that you just drank more than you needed to. I know if I forced down a quart of water first thing in the morning I'd get that "feeling" I'm talking about right away, no doubt about it.

Personally, I don't adhere to the concept of drinking a prescribed amount of water. I'll slowly down a half liter before hitting the trail, and then drink when I'm thirsty, but only as much as I feel the need to. I never force myself to drink more than that. That said, I'd say I feel the need about every 15 minutes or so when I'm hiking with my pack on, but I usually only drink about 250 ml at a time and I drink it while stopping for a short rest, and don't gulp it down.

I'll also second the "Emergen-C" packets. My wife and I do two of them every morning. I take mine after my morning coffee and before I eat. I mix both packets in about a half cup of room temp water, mix it until it completely quits fizzing, and drink it slowly. Within a minute or two I can feel it. It's not like "Wow! I can dance", but I can feel it, and I feel it even more if I don't take it.