Don't walk that far? Lol.
You shouldn't be testing her that much. If she hasn't learned to heel...doing it for long distances isn't going to teach it to her. Start slow. Heel for 2 steps. She's in heel? Reward. Three steps...reward. Four steps...reward. Five steps? She lost focus? Mark with a no, no reward, start again. Short distances, short times.
If she is treat motivated, you should just leave the treat out in front of her nose but hidden in your hand. She'll be nibbling at it and should completely be focused on the treat. She won't even realize she's heeling, then you reward after a few steps. Do this over and over.
I don't recommend heeling that far on walks. Training should be fun and if its a young dog, or you're just starting to teach a proper heel...walks aren't where you want to do that. Walks should be fun, not boring heeling sessions. If you want to teach a real good heel, use a toy, build up drive, play with your dog, and then quickly get her into a quick heel. Then reward... It's kind of hard to explain it without showing it.