The key to good exercise is work up the heart a good amount so that the dog reaches a good pant. This happens to be the dog's reaction to body heat, so it is a good indication they are working hard. If the pant is too strong, you will hear a harsh wheezing breath that sounds very unpleasant. Be sure to give your dog rest, especially older dogs and provide plenty of water when finished. They will be very thirsty.
This guide is based on my own experiences with my dogs, but due to the nature of taking care of pets mileage may very and suggestions are certainly welcome! Let me remind you that playing with your dog is not only beneficial for the mind and body of your pet, but also for you! Daily exercise is required and most ideally more than once in a day.