If your big dog is getting up in years, they may have slowed down a bit. They no longer run in circles chasing their tail nor spring into action every time they hear a leaf blow. That doesn't mean your dog doesn't still need exercise and mental stimulation or that they don't enjoy spending as much time as possible with you. They may not be able to go jogging with you anymore but they can do some less vigorous activities, and will enjoy the diversion. You may have to practice cutting back on some activities but there are still plenty of fun options for you and your large breed senior pal to explore.
The more active your dog remains the better outlook for the rest of their life. They are happiest when they are spending time with you and making you happy. Large breeds in particular need to keep up their physical strength to maintain mobility, so don't let your old dog lie all the time. Make these years the best years of your dog's life and share some time with your best pal.