Welcome to the world of bike commuting. Be it one miler like yours, seven miles each way like mine, or farther, a bike ride to work counts as a bike commute! And, as you seem to have already realized, even a one mile commute can become a ten mile commute if you know the right roads/paths!

My $0.02:

+1 for rolling up clothes as opposed to folding. I don't car so much about wrinkles, but if nothing else, I do find it easier to pack clothes in my panniers that way...

For a one mile ride, I'd just wear my work clothes. There's no need for speed on a one mile trip like that, so pedal soft and take your time... I go seven miles each way for work, so I bring a change of clothes with me.

BTW, Make sure to tuck our right pant leg into your sock or get an elastic band to hold you pant leg so you don't get grease all over it.

I also think a backpack is probably fine for a one mile ride, especially if you're not having to change clothes. A lunch and whatever else you might need for the day would fit easily and not weigh too much. I used one for the first eight or nine months commuting, and that was with change of clothes, shower stuff, etc. I have since gotten a rack and a rack trunk for my commuting needs, and they're GREAT!!! Something like I've got would certainly be more comfortable and convenient, but it's far from necessary and if it breaks the bank right now, then don't even think about it. If you stick with the commuting, have the dough, and want to get more equipment, go for it. Buying bike stuff is fun!

Anyway, welcome to the ranks of the bicycle commuters! Keep it up!