I wanted to do this, but DH is not recovered enough from his hospital stay early this month. I wouldn't go for the 50 if you haven't been riding long or much. My first organized ride was 50, but it was flat and I was in the best shape of my life from running 5+ miles often. The 20 miles sounds like a good challenge if you usually ride 30-45 minutes on a hybrid and it will give you a good experience of the ride.

Everyone has given you wonderful advice. Here's mine:
  1. Have fun!
  2. Ride at the pace YOU feel comfortable at.
  3. Stop for breaks if you get tired. And enjoy at least one rest stop, it is one of the unique things about an organized ride.
  4. Take your own water and snacks. They will probably have plenty, but you don't want to rely on the rest stop.
  5. HAVE FUN! And come back to tell us about it.