In practice, standing still is impossible with an 85-pound dog pulling me. What can I do?

In a straight battle of power, you're not going to win, so concentrate on training. Get your dog listening to you with several short training sessions a day. Work on "Heel" off the lead in your home. Find a super tasty treat and show it to the dog. Only give it when the dog sits (don't tell the dog what you want, have him work it out, however long that takes). Once he regularly sits without being asked, take a couple of steps and then stop. Most dogs tag along and then sit. Keep adding in more steps and stop before he jumps ahead. Now add the cue word "Heel".