Crux of the matter: It's better to have water and food and not need it than it is to need it and not have it. Sometimes the advice on these threads seems like it's all aimed at potential TDF competitors and loses sight of the fact that if your average ride is 10 miles, 20 miles is a pretty big ride. Distance is relative. While you're unlikely to actually bonk on a 20-30 mile ride, even if you're a newbie, you'll still be happier with lots of water and the option to have a snack at the halfway point.

BB

exactly... I know where I can get water on most of my rides if it becomes necessary...

https://scontent-b-dfw.xx.fbcdn.net/...39433051_n.jpg

this is my current setup on my "touring" bike... I only ran 35 miles like this (longest ride up to that point in my "come back") it had been pretty hot just a week or two before so I'd been used to using A LOT more water so I did this setup which comes out to about 3L... 1 bottle was some weak Gatorade mix... it was a mile temp ride so I only ended up using my 2 smaller bottles and a little from the big bottle... had it been hot out I would have used a lot more... I sweat A LOT and have always needed a lot of water to stay hydrated while exercising... even more if it's warm/hot out.

most of my rides I only take the two normal bottles though, along with the other stuff I mentioned... for water I never know how much I'll need... and sometimes I'll turn a 15 mile ride into a 40 mile ride... I like being ready for that if it happens

I keep a clif bar/power bar in the bike saddlebag in case of emergency. It has been very valuable on a few occasions.
They survive harsh temperature extremes and remain edible for a very long time.

Lets start off with, I have bonked in the past and I have received the free ride in the ambulance for dehydration. So, a snack goes with me. Right now I have dehydrated cherries and something else in the bag. In the spring and fall, I take two bottles for water and I know where I can stop on my trips. In the summer, I take a camel back and at least 3 bottle of water. I figured out that I go through life partially dehydrated and as out of shape as I am, I sweat out a lot of water as I ride. YMMV

When to bring food?
 

I can go a long way on a banana or two.
Being well hydrated before riding is important as well... as in the day before onwards.

It's always smart to carry along a bag of "gorp" (good ol' raisins and peanuts) as a quick food pick me up. With some gorp and some water you're always prepared to fight off sudden hunger.

this (for me too)

Fig Newtons are good, and more durable than bananas.