Thanks for the advice guys.

I have to wear business casual clothing at work, and I'm not biking to work in khakis or slacks, so I do need to change. Partially don't want to get 'em dirty, and partially don't want to smell like sweat and grime all day, partially just for comfort.

I've got a rear rack I use for groceries, but don't have any panniers aside from a big-bag type. Anyone recommend a good pannier that doubles as a laptop bag, or something else suited for walking a few blocks, can carry a laptop and full change of clothes?

And thanks for all the cold weather advice so far. I know how to dress myself for long days in the field working in it, with moderate exercise, but wasn't sure how to dress for short bursts of moderate exercise like this.