I highly suspect that either 1) it's muscle or 2) you're better hydrated than you usually are (not that you're retaining water, but that you're normally on the dehydrated side of normal...)
I certainly wouldn't worry about it - I frequently gain weight on the day after a longish ride. It does go away. To add insult to injury, my jeans are usually *tighter* around the stomach - something about the position on the bike or ??? Who knows - it too goes away. It sounds like your intake was about right for a ride of that length - I certainly don't think you over did the food side of things.
Keep pedaling - that's a heck of an inaugaral ride!!!
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