I bike in skirts very often, and usually don't wear anything special underneath them. A-line is indeed the best - the other day I rode about 20mi/30km in an a-line skirt.
To minimize flashing incidents I weight down the front of the skirt. If I'm wearing a lightweight skirt I will clip a binder clip (like
these) to the center front hem of my skirt. This makes the skirt want to hang down between my legs rather than blow up around my face. With, say, a heavy, lined wool skirt, I don't usually bother doing anything; it will stay down on its own, and anyway I'm probably wearing tights if I'm wearing a skirt like that.
To minimize skirt-caught-in-brakes-or-spokes incidents (only a problem with longer/fuller skirts), I bunch up the front of the skirt in an elastic band (preferably a rubber band, not a hair band, as hair bands can be too slippery), to make what I delicately describe as a "skirt-wang". One of my bikes has hub brakes, so it's impossible to get the skirt caught in the brakes. That bike also came with skirt guards but I'm not crazy about the look of them so I haven't bothered with them yet; the rack and fenders act as a sort of rudimentary skirt guard, and anyway I have only had problems with getting caught in the wheel at the half-circle skirt level (and every then it was below-the-knee).
I'm biking similar distances to you - my commute is 12 km - in my regular clothes, which are mostly off-the-rack (all of my trousers are off-the-rack, and not especially high-quality), and I haven't really had any problems with wear. I also have some inexpensive workout pants ("performance polyester" kind of stuff from TJMaxx) that I wear on the bike if it's very hot or very cold, and they don't wear out either. Honestly I think walking is probably more wearing on my trousers than biking - when I walk, my thighs are rubbing against each other, but when I bike there's just a little rubbing against the seat (which is much smoother). Your thigh gap may vary :P My boyfriend wears the Levi Commuters and loves them (and they are good-looking trousers).