I am 65 and I used to feel like you and gave myself a full body, mind and soul MOT!!
I got my thyroid sorted out, (I was very low) this helped my tiredness ( am now on levothyroxine) and I went on an anti inflammatory diet because of my arthritis which has stopped the flare ups!
I practice fasting once a week and in addition to dog walks I swim, surf, I got myself a pony 😅 so everything to do with owning one keeps you fit and toned from grooming them to sweeping the yard to opening heavy gates, mucking out and getting on and off as well as riding them) you don't want anything too big as you won't be able to get off and on easily, mine is a perfect 14 hh and a good doer so easy to keep. She has changed my life for the better!
I also go on long runs once or twice a month when the conditions are welcoming.
I also have started indoor rock climbing ( bouldering) because the centres usually have long opening hours so you can do this if it's too late or in poor weather. Perfect if you work full time.
Indoor rock climbers are insanely toned and slim, start off doing the beginner 'rocks' which are easy but help develop your confidence and strength gradually but definitely, every time you go you will notice a difference so don't worry about not having achieved much each time. The build up is insane over even a few days!
The reason why bouldering is so addictive is you only compete against yourself and you need to engage your mind with hand, eye coordination as well as complete body strength, not just arms and legs.
It completely tones your core, back, arm and leg muscles, and is proven to be wonderfully helpful for your mental health.
I didn't enjoy it the first few times I went but then I noticed a huge difference in strength every time I went and so I have gone up a colour boulder stage which I am very proud of.
I also like to go skiing 2-3 times a year.
For me, a combination of fun things to do is key as I get bored with routine.
Good nutrition is key, lots of fish and green leafy vegetables, pickles, miso, nuts, seeds and beans but not being religious about it.
It's about mind, body and soul conditioning and happiness, it's not about focusing on just one thing which is why I think focusing on diet without thinking about your mental health won't work..
Do things that make you happy, for example, if you love horses but can't afford to ride then find out where your local Riding for Disabled centre is and start volunteering. There are lots of people of all ages who help out and it's very sociable and every thing to do with horses keeps you fit. If you become a regular you probably will get to help exercise the ponies as they are often only used in term time.
Good luck with whatever floats your boat but have a health check first 🥇