I worked in the PO for 27 years. The letter carriers swear by these.
http://www.walmart.com/catalog/produ...i_sku=10910809
I crack one at the start of the hike and its good for 6-7 hours. They can make your hands clammy though if you keep them in all the time. If your hands are good just zip it in a pocket. If you can keep your hands from getting cold in the first place it is a much more pleasant experience. It took me a while to learn and I still have issues with numb fingers, but these help tremendously.
KDT