When I was growing up I was never in to sports or exercise (something I've tried to encourage with my kids, I think that's really important). While I was lower normal BMI, I was totally unfit and sedentary.
Recovery after serious illness/treatment in my 40s got me to start looking at exercise. I tried gym didn't like it, tried running and hated it.
For me I'm deliberately a lot more active in the course of my day. During the week it's small/silly things like parking far away in car park, using the downstairs coffee and toilets in work, always use stairs and never the elevator, getting off the bus one stop earlier, always going for a short 15 min walk near my home in the event. It adds up!
At weekends I deliberately put some extra physical effort into the things I normally do (some gardening, housework, cleaning, sex
, etc.)
I'd also try fit in a longer 10k walk at weekends with DH or a good friend.
End result is 8-12k + daily steps with no formal exercise.