Whatever bit of fitness you may have - start honing it. Honestly, the massive physical slide didn’t start at 40, that came later, but if I’d started working out and really paying attention to my diet at 40 maybe I wouldn’t be the round ball of blubber that I am now. I say this as a guy who was fairly active and fit during most of my twenties to early thirties.
Whatever bit of fitness you may have - start honing it. Honestly, the massive physical slide didn’t start at 40, that came later, but if I’d started working out and really paying attention to my diet at 40 maybe I wouldn’t be the round ball of blubber that I am now. I say this as a guy who was fairly active and fit during most of my twenties to early thirties.