Though I wouldn't go to a gym, what Henya says is right.
I go power walking every day. I'm fashioning myself on a Sydney woman, Heather Lee. She is 95 and wins lots of power walks, beating women far younger than she is.
I was a distance/marathon runner, and gave up because of my knee pain. Later I discovered, after taking every skerrick of processed food from my pantry, the knee pain had gone. It has never returned.
I walk every day, fast, I would never go to bed without stretching, and when I remember, I lift wights.
We never do takeaways.
We only eat out occasionally, and that has to be at a trusted venue.
I cook everything from scratch, which means I have one cooking day, where I use my slow cooker,(curries) and my tagine,(middle eastern or lamb shanks) and prep other easier meals like a Paleo meatloaf, ready to pop into the oven. I also make my own keto bread.
This may seem a lot, but it's only a couple of hours' work.
I'm not saying for one minute that I could easily don a bikini, but I try to stay focussed ad happy. Nobody can escape growing older, but small but significant changes in how we eat and what we are prepared to eat, will make the journey easier.
The wonderful Heather Lee.
https://www.abc.net.au/news/2023-01-01/96-year-old-race-walker-heather-lee-keeps-smashing/101820380