I'll chime in on this. I do think genetics have a lot to do with it.
My wife is IN shape. We're apart right now and I just talked to her on the phone. She just ran a marathon today for fun. Like there's a marathon today in the town we live, so she just was like, "I've never ran one, I should." So she did her own route that was more uphill. haha. She regularly rides her bike 100 miles. One time she was on a trip and I told her about a restaurant she should go to. She was bored so she walked. 23 miles each way. She's insane.
She's also pretty overweight. Like if you saw her you'd think she was a typical fat lazy American. She doesn't go to a gym....she likes walking, hiking, and biking. She's great on hunts as she loves camping and likes carrying heavy loads in a backpack. lol. I really don't understand why she is overweight as she eats literally 1/3 of what I do and is 20x more active.
On a side note, if you are a runner or whatever. Her feet bother her sometimes. I cajoled her into just buying new shoes every couple hundred miles and it's made a huge difference. Expensive, but yeah. I just took a literal contractor bag of new looking altras into goodwill before I left.
Since I'm talking about her, someone will laugh at this: When she was 19, her and a friend just up and took off from Boston and biked across the country in a big circle. They ended up only spending about $200 each on candy bars and such. The stories are hilarious. Everyone was all about helping these two young women when they were out in the middle of nowhere and feeding them. My favorite is from somewhere in southern Louisiana. They knocked on a door to ask if they could camp. A woman opened the door, they asked and she was like, "No, no, Honey. No. This won't work." So she washed all their clothes right then and made them shower. She gave them robes to wear. She made her husband take them out in his air boat (in robes) to go catch catfish, and made a literal feast of catfish, okra, gumbo, the whole 9 yards. She also made them call their parents to check in and let them know they were OK. She also had them sleep in her guest bedroom. Half the people they asked to camp fed them, so they spent like no money. This would have been a bit over 20 years ago, so this kind of thing is more popular now and I don't think you'd get the same reception. If you want to get in practical shape, bike across the country. haha.
Talking about practical fitness reminds me. My dad had elbow surgery when he was 60 due to shooting a bow. He's super picky with doctors and went to an orthopedic surgeon who basically only deals with professional athletes. With the surgery he had done, you couldn't mess it up by overuse, but needed to use it. He had a bunch of time off work after it.
The surgeon and physical therapists were totally dumbfounded how he rehabbed so quickly. The surgeon was like, I deal with football players throwing millions into rehab who are in their 20s and they don't recover as fast as you with this same surgery. How did you do it?
My dad's surgery was in late May. He basically picked berries every day to make jelly and went catfishing the rest of the time. There was probably some busch light drinking involved too. I think there's something to be said about being active in general and not just sitting at a desk or at the computer and then going balls to the wall at the gym.