[header] How to tell if your older dog is over exerting itself [title] Look for excessive panting and trouble breathing. [step] If your dog is getting winded more easily than it used to then you may need to exercise it less. Problems breathing, coughing or hacking, or lots of panting can be a sign that your older dog is doing too much. 
Allow the dog to catch its breath and calm down. After the dog has recovered you may be able to continue the activity at a slower pace.