I don't drink as much as I should on the bike, especially when I'm in a fast group and it's all I can do to hang on. If I take a minute to grab my water bottle, I'm off the back. I make myself drink every 15 minutes. I also make it a habit to weigh myself before a ride and then after I'm done to be sure I've taken in enough fluids. You should weigh more after your ride. If you weigh less, you haven't taken in enough. Another sign that you haven't hydrated enough is if you don't need to pee after a ride---especially on hot days.

Congrats on your 20 miles! That's quite an accomplishment. Keep it up!