Katie-
You will probably be fine. But do drink plenty of liquids while riding that 50 miles, and eat a snack in the middle of it- a few fig newtons, granola bar or something.
By the time you actually FEEL thirsty, you may already be in a serious fluid deprivation state that can make you feel suddenly sick and stop your ride cold.

And remember, you can STOP when you want to if you feel totally wiped out. What counts is not that you finish the 50 miles, or that you kill yourself trying, but rather that you are making the effort, doing a really healthy fun activity, and creating a great lifestyle! Whatever distance you can do that day, even 20 miles, it simply means you are giving your best shot. That WAY beats all the kids who are just sitting on their bums at home in front of a tv or computer screen!