Topical anesthetics hit leeches hard. Their entire body is a mucus membrane, so they absorb them fast. I'd use tobacco infused vegetable oil and benzocaine spray.
Mix some vegetable oil and snuff into a paste, and before you go out, cover your knee-high socks with that tobacco paste.
Wear a 2nd pair of socks over the treated ones, and tuck the hem of your pants under this 2nd pair of socks.
While you are out, spray your boots, socks, and pants with benzocaine topical anesthetic once every hour or so.