A fit person can run and slow run for quite a while. The best and most easy metric is to use BMI, its a calculator, you can find it on google. It uses your weight, height to figure out your BMI. If your BMI is under 25 you are doing OK. Fitness is also relative, if you want to start with some bar. Think PE minimums, 10 pushups, 10 situps, 10 squads, 10 minutes run, touch your toes.