Thanks for all the comments. I was expecting to see more people hydrating before or after but it seems like that is not a thing.
I think my issue may be more that I am not drinking enough water in the latter part of the ride. On cool days my two bottles are more than enough, but when I am sweating like a pig it might not be. I also sometimes use one of those quart bladder bottles which fit nicely in the middle back pocket, on a really hot day I will start them completely frozen and will be drinking them at the end. I probably should be doing more of that, I would have more than enough water for three hours with such an addition.
I'm not concerned that I am getting seriously dehydrated, I am drinking all along, don't feel particularly thirsty, and don't have any of the symptoms such as darker urine.