My dog is getting on in her years, rapidly approaching her 9th birthday, and she doesn't get as much exercise as she used to. When she was younger, she and I would enjoy a 2 hour walk nearly every day, rain or shine, and now she's happy to get out a few times a week for about a 45 minute meander. While older dogs may not be as active as they used to be, they still need exercise, and here is how to do it.
Pay attention to your dog. Your dog will tell you how much energy they have. Some dogs never appear to get old, while others show their age in their shoulders and hips. My dog will begin limping and holding her head down, walking with a stiff gait in her shoulders, if she is out for more than a half hour or so. This is particularly true in the winter or colder months. Your dog will tell you when they are done getting their exercise in their slowing gait or even lying down in exhaustion. Don't push your dog to walk more than they used to if they are showing you they just want to go home.
I prefer to drive to a nearby canal and let my dog meander up and down the canal until she lays down or comes to my side whining that she wants to go home. Driving to a destination where you will be walking your older dog (like a park or bike path) rather than walking there ensures that just in case your older dog decides they are done walking, you don't have to put them through the agony of walking home again. This method has come in handy many times for me, as I can't lug my 80 pound mutt home, and I certainly don't want to make her stagger her way back to the house when her joints stiffen up. Having your vehicle nearby (or only walking your dog up and down your street or around the block a few times) allows you to have easy access to home when your dog suddenly wants to quit walking.
Try to avoid walking an older dog in super hot or super cold temperatures. Older dogs react poorly to hot summer walks and freezing winter walks, where they become dehydrated and exhausted much more quickly. If walking an older dog (or any dog) in harsh heat or cold conditions, watch for signs of discomfort, like shaking in the cold or holding their paws up (dogs will do this in both heat and cold, as the pads of their feet are uncomfortable due to the weather), or extreme panting and sluggish movement in the heat. I prefer to walk my dog, once again, on the canal in the summer months, where she can get a dip in the water to help keep her cool.
If your dog isn't up for it, walk them every other day instead of every day. My dog sleeps the entire day after her walk the day before, and her walks are 45 minutes or so, sometimes an hour, depending on the weather. She is perfectly content to rest up the following day for her walk the next day. Watch for signs of limping or discomfort after walking an older dog, and get them to a vet if their walking becomes troubled or worse after a walk.
If you are concerned about walking your older dog, have them checked out by your vet and allow your vet to help you choose an exercise plan for your older dog. Older dogs enjoy that change of scenery when they walk, and it helps to loosen up those aging joints and keeps them in shape, and getting the go-ahead from your vet allows you to feel more comfortable in walking your dog if your dog just seems too old for their walks anymore.