At 75 I do almost everything that I've done for my entire adult life. I stopped running on purpose, and shifted to interval training on my stationary bike, biking, hiking, lifting weights, stretching, golf, bowling, and taking daily naps.
Because some of these things can be measured, I know exactly how much I am slowing down. At 50 I was driving a golf ball 240-250 yards, and now 200 is a good drive. I ride my bike at 10mph rather than 15, I walk at 2.8mph rather than3.5, and so on.
It isn't graceful, but I can accept the deterioration without too much complaining.