My feet are the limiting factor on any long bike ride under 40F. My feet have a very fine line between painfully cold and sweating. On the bike, they start out hot and maybe sweating and then get painfully cold after about 45 minutes. I've tried thin socks and vented shoes (to prevent the initial sweating), sealed shoes, bags, wool socks etc. I have not tried those chemical packs or outer shoe covers. The ONE thing I've found that does work although not convenient.. Get off the bike about every 30-45 minutes and walk around for a few minutes. Odd, my hands are the opposite, start off cold and eventually warm up.