It's not hard, many different pants and combinations work. I like a soft stretchy ("softshell") pair of looser pants that I can layer underneath as it gets colder (current and probably all-time favorite: adidas Trackstand). Cross country skiing pants also work well. The pants are comfortable up to transition to shorts weather (about 50F for easy running) and down to 25-32F or so depending on how easy I'm going. Colder, than that I'll add one layer at most, choosing from thinner to warmer: regular running tights, various different long underwear (the knit Craft with grid pattern is really warm for its weight) to thick, stretchy fleece long underwear tights. That's good to as cold as I'll go out, -10F or -15F or so. Below about zero F, add wind-front underwear if a guy (the softshell pants are wind resistant enough above that).