If he's really out of shape, make sure to give him time to get into shape before you ask too much. http://www.olddominionrides.org/EndurancePrimer/11.html
has a lot of good information. One of the biggest challenges you may face is that he's may overheat easily to start with because his body has to adapt to get rid of heat efficiently, which is going to mean you have a hot, wet horse that takes forever to try and must be kept from getting a chill or tying up in the cold. If you watch him and ease him into it, you should be fine, but I wanted to bring it up as it's easy to forget how hot they can get when it's so cold outside!
My good friend told me that generally speaking, it take a year or two to get a horse into shape for full-distance endurance since their feet and bones take so much longer to adapt and condition to the task than their heart, lungs and muscles. Too much too soon ends up in lameness. Just finishing a limited distance might be a good intermediate goal you can accomplish sooner though!