The correct answer to 'how much walking is too much walking for my dog' really depends on your dog. Breed, age, health, age, etc. etc. Other external things like temperature also play a part.

Without some of that info all I can offer is a generalization that you should pay attention to your dog and how she reacts to your walk durations. If she seems like she is getting tired, you might want to tone it down. You can work up to longer walks by starting with shorter ones and gradually increasing the duration.

Always be sure that your dog gets plenty of water.

Also be sure to keep an eye on your dog's pads. Look for cracking or dryness that might result from too much walking on sidewalks/roads.