I'd ditch that mostly cotton underwear. If it gets wet you'll be in big trouble, specficially hypothermia, because cotton absorbs a lot of moisture and is slow to dry. Just getting sweaty could cause a problem. There are lots of inexpensive sources of breathable synthetics. Look in the athletic departments of big box stores for starters. Thrift stores are another source. Just check those fabric tags to eliminate cotton.

I love to wear cotton at home, but I leave it there when heading out!
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May your trails be crooked, winding, lonesome, dangerous, leading to the most amazing view--E. Abbey