A running skort is good, but I cut out the shorts liner. My favorite, though, is one I made out of men's swim trunks, again cutting out the liner. Lightweight, quick-drying.
I like a skirt because:
- It's airy.
- You don't have to undress to add or subtract warm layers or rain pants.
- Nothing tightens around your legs when you take a big step up or down.
- You don't have to strip and squat to pee, or even take off your backpack.
My hiking skirt is black, so black microfiber underwear looks like it could be part of the skirt if someone below you on a climb happens to look up. I also like boxer shorts, for modesty and ventilation. Men's medium work just fine. Again, with some maneuvering I can just pee out the leg, although it's not as easy as with elastic-leg underwear.
BTW, the men's swimsuit to skirt conversion is so easy I did it on the trail with my minimalist sewing kit on a "nero" day ("near-zero", hike in the a.m., take the afternoon off for chores and rest.) Cut out the leg-to-leg crotch seam, trim excess fabric a little longer than leg hems, sew existing seams to incorporate any extra fullness, trim seam, hem, you're done. Half an hour at most. I bought the men's trunks on sale at Trail Days in Damascus, VA when I went through, because I'd gotten too small for my running skirt with a 10-pound hiking weight-loss. The men's suit has an elastic waist and drawstring, to accommodate further loss (or post-hike gain.) That was in 2009, I wore it from Damascus to Harpers Ferry, WV, have worn it on section hikes from there to the AT midpoint in PA, and will be wearing it on my midpoint-to-Maine hike this summer.