She finished the Marathon in five hours. This is her hobby, her fun. Every person is different, however, so according to your physical capabilities and interests, choose something that is right for you. Inevitably, you will want more of this. All you have to do is make the choice.
Balance is the key.
Not too much and not too little. Exercise does not have to hurt to be beneficial. An increase in adrenaline, circulation, positive brain chemicals and oxygen does not hurt. So have fun. Take a walk with someone or alone.