How much to eat and drink during a 50km ride?
50km isn't that enormous of a distance - and especially for someone who does 18km twice a day.  It sounds like your current pace would have you completing that in about three hours.  I'd bring a snack or two if you're worried about being peckish, but unless you're going all-out for a personal record or you skip breakfast you should have no issues.  Just keep turning the pedals, make sure you drink some water, and feel free to take a 5-minute stretch and snack break if you're feeling tired.