I typically lose weight on long rides as well, but it usually takes a couple of days to register on the scale. You will probably lose any weight you gained once your system gets back to normal.

50 miles is a great accomplishment!!! You should be very proud and I would be surprised if when you look back on this in the future you don't remember it in a postive way. Funny how time makes your forget the pain and only remember the "highs" of the ride.