How do you manage to commute 20 miles to a job in an office environment and not have to change clothes anyway? Most people with a lengthy commute wear full-on cycling clothes (they are more comfortable) and change when they get there. My commute is only about a third the length of yours, and most of the year it would be quite impossible for me to do it in office clothing, unless I rode VERY slowly (and what fun is that?).
I use these, pulled over my normal cycling shorts... http://www.rei.com/product/754988#prodInfor
They're not tight, not loose, somewhere in the middle. And in fact I just keep them in one of my panniers, and walk out to my bike each morning in cycling shorts and make the call when I get outside. They work very well (for me), and I often go inside restaurants, coffee shops, etc without feeling like I'm wearing geeky tight. More like a workout pant.
Since I don't like to come into the office in bike shorts, I usually wear a loose pair of nylon sport shorts over my bike shorts. For cold weather instead of tights I prefer close fitting running pants made by Sport Hill. They're close fitting and elastic at the ankel but not form fittings such as tights As an ex-runner I've still got several pair and these are "less" inappropriate.
I wear fleece sweatpants over my bike shorts with reflective leg bands. They keep my legs warm, and they block the wind. The leg bands keep my pants from blowing around, plus they help with being seen.
Remember, you are commuting to work. Not a weekend group ride.
I have a favorite pair of North Face soft shell running pants, which I wear over cycling shorts or bibs, and use a leg band to keep the cuff out of the chainring. A bonus is these pants have a reflective stripe down each side to help visibility.
+ 1 on the North Face runners pants. . .I wear them with temps in the 30's over regular shorts. They're not tight but not baggy, good windblock and warm enough if the other parts that get cold are taken care of (feet, hands, head. . .). They're loose enough to look "normal" to non-cyclists. . .
I walk in my office in full on cycling gear. Cleats, spandex, helmet,etc. I walk right past the security guard to the gym where I shower and change. No one has every questioned the 2 minutes I spend in the office out of dress code between my bike and the gym.
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