Q:

How much to eat and drink during a 50km ride?

I want to try a 50 km ride for the first time, to be completed continuously one morning.
The most I've even done before is an 18 km (70 minute) each way commute (out in the morning and back in the evening).
When I commute I eat (breakfast, and late afternoon snack) before I ride, but I eat and drink nothing during the ride.
I guess that the difference between that and a 50 km ride is that, on the multi-hour ride, I ought to eat or drink something on/during the ride to replenish? How much should I eat and drink then, and when? Should I expect to take rest stops, and/or to go more slowly on average?
I'm wary because I think I've read that glycogen depletes at around the 2-hour mark, whereas my longest continuous ride to date has has been only 80 minutes.

A:

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.

A:

This link "Quick note: Eating is the key to long distance biking" says,

If you don't eat, you have an hour, maybe two, of energy stored up.

Fortunately it's prescriptive too, saying,

And what should these calories be?
  Well, something easy on your stomach
  and fairly light. There are special
  sporting-related products that are
  generally right around 100 calories
  and super-easy on your stomach, but
  you don't have to go overboard. A
  piece of fruit or some candy or even a
  can of soda will work just fine. Just
  try to keep it around 100-200 calories
  maximum per hour.

... so that answers that: only about 100-200 calories an hour. Apparently you don't try to to keep up with the approx 500 calorie/hour rate at which you're expending calories.
As for what, this recommends bike-specific stuff based on maltodextrin, brand name "Hammer", and says that the body can't absorb more than 200-300 calories an hour - but that's for a multi-day ride.
Whereas this recommends:

Junk food (donuts, gummi bears, etc.)
Chocolate
Nuts (or peanuts)
Olives (salty, light-weight, and high-calorie: though I'll guess slow to absorb)
A Subway sandwich but without any hot/spicy components
Ice tea (sugar and caffeine)
Fruit (bananas, banana chips, fig bars)
Cereal bars

To answer the "how much to drink?" question:

This says, that 'one litre per hour' is on the high side, that that's for endurance not recreational rides.
This says 16 oz / 45 minutes (~ 0.63 l/h).
This says 28 oz / hour (~ 0.83 l/h).
This says a couple of big swallows every 15 minutes.

One of those articles also says that you thirst after dehydrating 1% and suffer when you drop 5%. I weigh 77 kg so I suppose I'm 46 litres of water, so 1% is about half a litre. If I don't drink when riding then (if I ought to be replenishing up to a litre an hour) that predicts that I'd be thirsty (1%) within 1/2 hour or an hour, and suffering (5%) within 2 1/2 to 4 hours.
This says that "Sports drinks have low-sodium levels in order to be appetizing to the general public" and suggests 500 to 700 mg of sodium per litre. Salt is 40% sodium, so that's 1.2 to 1.8 grams of salt per litre.
1.2 g salt is about 1/2 cc or 1/5 tsp (per litre of water), which is less than I'd imagined.

A:

Based on the answers provided thus far....
 - Moz suggests:_ Cycling knicks, water bottles, and a puncture kit. Excellent advice. And, I assume that Moz means padded cycling shorts. The puncture kit is also good advice since you might get a flat between stops. Water bottles? Seriously, who can cycle without water bottles?
  - The test ride: Equip yourself with some calories and at least 2 water bottles and the puncture kit. The puncture kit is good since no one will be out there to help you on the test ride.
  - The ride. You have no worries. 
And @Chris -- You're already building up endurance miles. You'll be fine. You'll sail through the 50K with no problems. The 50K Ride is not "long distance". 
Anyhow, the real answer depends upon your current cycling fitness level. So, the real answer will be quite different between a beginner, vs, a fitness-cyclist coming out of the winter doldrum, vs, a racer; and etc... 
The ride you intend to do in June is a supported 50k ride. I looked at your ride map and there are 4 rest stops along the way. You really don't need to worry much about hydration or consuming enough calories. Take advantage of the rest stops and you'll be totally fine. You already have a good start with your daily commute. My guess is that you'll do much better than you think, since you won't need to worry about stop-lights, etc. 

