How to improve (physical and mental) stamina?

How could I get stamina?

Build your ability to focus. Your brain isn't a muscle, but it can be strengthened like one. Work up your ability to concentrate and focus on serious work over time in the same way you would build strong muscles. Gradually increase the duration and intensity of your mental tasks.

 Fáradtság, kiégés - fatigue, burnout
stamina - állóképesség, kitartás

http://www.wikihow.com/Improve-Stamina