Excessive walks for my dog to poop

Question
I rescued my 40 lb. male (neutered) lab/shepherd mix 3 1/2 years ago.  I began walking him immediately and did not give him a chance to poop in our yard.  I've been walking him for between 6 and 10 miles a day ever since.  How can I retrain him to poop in our yard.  He will pee in the yard, no problem.  I've tried just taking him in the yard and he simply won't go until we walk.  It's winter, cold and nasty outside and I am tired of walking for him to poop.  If he does learn to go in the yard, I wouldn't eliminate his walks completely.  Help!
Thanks,
Elizabeth Feeney

Answer
The answer to this question may seem cruel, but just dont walk him. Take him out around 20 minutes after he eats in the yard and say "go potty" . He may not go..try again 15 min later and continue until he goes. All it will take is him going a few times and getting praise for it, for him to figure out that he is supposed to go there. He will HAVE to go eventually.
Thanks and good luck
Nancy