I bike to work a couple of days a week. It’s five miles, but I live on the top of one hill and my workplace is on the top of another, with a valley in between, so it’s a workout. I carry my clothes in panniers and shower and change at the office.
Ditto what everyone has said about lights and visibility. My rule: if I have to ask myself, “Does that guy see me,” I assume he doesn’t.
In addition to blinky lights front and back and a reflective vest, I also have one of those strands of electroluminscent wire that I wrap around my frame and an LED on one valve stem. I figure anything that makes a driver think, “What the hell is THAT?” is good.
If you wear glasses, get the kind of rearview mirror that mounts on your temple piece. I never ride without one. It’s so much better than the kind that mount on your handlebars.
I was using an inexpensive set of panniers from Nashbar, which were OK when the weather was good, but I bike in the winter, too, and my hands would be frozen by the time I dragged my shit out to my bike and got it all packed. I just bought an awesome pair of panniers that I can mount on my rack in about 10 seconds, so I can pack them inside. I love 'em.
You can get a pannier that is a full-sized garment bag, incidentally.
If you’re going to ride when it’s cold, shoe covers are great! They really keep my toes from freezing.
I keep my bike in my office, too. I hang it from ceiling hooks.
P.S. Holy cow, runner pat! I knew you’d had a serious injury but I’d never read the full story!