Why is there a chance of breeding her?

Is she a papered purebred? If so, is she an outstanding member of her species? The only reason to breed a dog is to improve the breed, and even then should only be undertaken by reputable breeders.

If not, there's no need to breed, at all, and many, many reasons not to... not least of all her marking. You're shortening her lifespan and leaving her open to a lot of possible problems and complications by not having her spayed.

You can't break a dog of a habit that is so second nature. "punishing" her by not taking her for walks won't do any good. Just be patient! Do you have somewhere to be, when you're walking her? Just let her enjoy herself, if she's not hurting anything.

As an alternative, follow what someone else suggested and take her for runs, not walks. Both of you will get better exercise, and she'll probably have more fun.