Though obesity doesn't necessarily cause diabetes in dogs, many veterinarians believe it can lead to the health problems that lead to diabetes (like pancreatitis). If your dog already has diabetes, exercise helps by utilizing energy (glucose) and can aid insulin therapy by increasing blood flow which can help improve insulin absorption. So, not only is it important to regularly exercise your dogs to help prevent diabetes, it can also help lessen the effects of canine diabetes. 
 
Just like in humans who have diabetes, maintaing a healthy
body weight can help cells better react to insulin. If your dog
has been diagnosed with diabetes, maintaining a healthy
body weight through a regulated diet and exercise is one of
the most important gifts you can give your dog. A healthy
weight and body mass index is highly variable depending on
breed, age, and gender of your dog, so it is best to ask your
veterinarian what a healthy range is for your dog.
 
Once you understand the healthy weight range for your dog, work on scheduling regular times to exercise for your dog. It's best to exercise with your dog a few hours after their last meal. If you wait too long after a meal, too much exercise can lead to hypoglycemia, which is a dangerous drop in blood sugar. If left untreated, hypoglycemia can lead to unconciousness and even death. 
 
Exercise Tips for Diabetic Dogs
 
1) Just like with meals, it is best to keep a regular exercise schedule in terms of duration and difficulty. A diabetic dog's body is much more prone to blood glucose swings because of their inability to regulate their blood sugar, so it's best to keep a consistent exercise regimine. Figure out an exercise that works for both you and your dog and stick to it.
 
2) Walking and jogging are the best forms of exercise. Though it may be fun bringing a ball to your favorite park and watching your dog sprint after it, the stops and starts associated with sprinting can be difficult for diabetic dogs. If possible, keep their exercise to walks and jogs.
 
3) Avoid off-roading. Diabetic dogs are more susceptible to non-healing wounds and infections, and thus it's best not to encourage them to get off the trail where they may be able to sustain cuts or worse.