training.
Weight lifting is the best and most sustainable way to be fit.
I'm not talking isolation exercises or machines or anything like that... Compound exercises that work the entire body.
Even if you're lifting in the wild.
The more heavy things you lift. The stronger and more "fit" you are...
If your sister stops running (which is pretty dang impressive), then her fitness will start to deteriorate faster than someone who has been doing resistance training.
Plus, the wear and tear of cardio is another story.
Nutrition was always a given.