I would like to add my two cents!

I agree with Zen, as to me - a 20 mile ride IS LONG. Im frail, still and although I would love to be able to get by with a banana and a bottle of electrolytes, it takes me a while to cycle that. I cant eat at cafes due to allergies so have to carry food stocks, like Im riding bush rides.

My work commute is about 45 - 50 min ( about 25 km) but I add for that( Im a pack rat linus person anyhow!)

codeine ( I get migraines too)
sunscreen( a nifty tiny tube, for lips ears and nose, as we have high skin cancer here)
contact lens solution( as my eyes turn bright red and scare the cafe people without it.)
a towel( to shower before work)
whatever work clothes I need
wallet( with too much crap in it.)

So its pannier time.

I do agree with the race chick, less can be good. Im working on that!
but I think taking bike into shops is a no no here( as much as I like the idea of not letting it out of my sight) the Mall is gonna have other ideas, sadly.( I mean , why shouldnt I take my bike? People take their prams, and wheelchairs but I digress..) A lock is essential sometimes. I do admire someone who breaks the non bike barriers, good on ya!
I dont carry patches, rather tubes as my best riding bud, said - those dont work so well for road bikes. Besides tubes are cheap.
Sadly, I get cold, even in summer so I am always carrying around heaps of clothes I am shedding and re-wearing, Its annoying but from a virus so cant be fixed. I was once insulted by a cycle shop mech who sneered at my pile of jumpers in a pannier , on a 28 km ride. I had a two hour wait,later at a friends and wore them all later that day. I would love to be someone who sets off like those gorgeously fast riders in packs that whip around in shorts and a jacket, on frosty mornings( and ride my commute in like 25 minutes!)

I also had a handsome guy rider give me a spare tube cap. People who have spares rock, if your one of them, I love you! I have ADD and I KNOW someday I will lose my rear ,even clutching it with both hands so someday, I may need you kind people!
And racers, someday I hope to be like you . Lean,mean riding machines with a banana and bottle of water( and NO BAGS!)