There are some great products to use if you have difficulty walking your dog on a leash. Dogs tend to want to pull when walking them because it is a self-rewarding behavior. They pull and they are rewarded with moving forward. With proper training you can train them to stop pulling. But I have found with large dogs its also helpful to have a harness on their head to keep them from dragging you down the street, it won't necessarily fix the problem but it will give you leaverage to use against their brute strength.
The premise is that you lead them with their head. Where their head goes their body must follow. It's the way they have been leading horses aroung for centuries. It doesn't take great strength to turn their head.
The harnes doesn't restrict their breathing, they are able to pant, drink water. It's not a muzzle, it won't stop them from biting but it will give you more control of their actions.
Two of the best products to use are the Gentle Leader or the Halti. Both are great products. I have a Halti for our larger dog Tanner. I think when I bought it about 6 years ago and it still works great. I also bought a Gentle Leader about 4 years ago to use on one of our other dogs it works well too. The Gentle Leader is a little less confusing to use, while the Halti is designed differently with more straps so it's not as straight forward. I think they both work equally as well. I would highly recommend using one with large breed dogs. I use to hate to walk Tanner, in the days before I understood more about dog training. I could hardly physically hold him on a walk. By the end of the walk he would be worn down enough that he would walking by my side by I would be physically exhausted. I couldn't believe the ease of walking him with the Halti.