What are you walking her with? What sort of collar/leash?
My primary exercise is walking my two dogs. They get a few minutes in the beginning of the walk to sniff and squat (as I like to put it) and a few minutes at the end. During the middle they need to keep pace with ME!

It takes a few days of being firm, but I've found that my dogs very quickly learned the routine and don't veer off the path very much at all any more.
That being said, I do have a few suggestions.
1) Look at what you're walking with. Those retractable flexi leashes aren't good for exercise-walking purposes. A leather or nylon 6' leash is what you need. And look at what is on your dog. Both my dogs go in standard chain "choke" collars. Don't let the word scare you -- they don't even feel the collar unless they are pulling. I used to use a Gentle Leader on one dog but he started to get a rub on his cheek so I stopped using it and now like the choke chain better. A Gentle Leader type thing (almost look like a muzzle if you're not sure what you're looking at) can be a godsend. You're controlling your dog by its face (sort of) which gives you a lot more control. Their necks and chests are burly and strong, but if you can turn its head, you can redirect the dog without much effort.
2) Don't be afraid to drag the dog along. Like I said, it's better to be TOUGH for a few days and get your message through than spend weeks and weeks fighting little fights.
3) Get a doggie backpack. I'm serious! I learned this from that dog whisperer guy and he is 100% correct! I would have never believed it if I hadn't seen it myself, but my lab mix walks now wearing a doggie backpack with a 3 pound hand weight in each side. He also carries water if I bring it, and empty poopie bags. A dog can be trained to carry 30% of its body weight (my don't doesn't carry nearly that much). But when that backpack goes on my dog is SO MUCH more responsive and obedient... It gives him a job -- a purpose -- and it makes a WORLD of difference!
GOOD LUCK!!!!!