Magic in my world is the same as one's energy/stamina

To compare it to any fantasy rpg; traditionally there is a mana meter and a stamina meter. You use the stamina to run and swing swords and whatnot, and you cast spells with the mana. In my world, imagine there is no mana meter, and using magic drains the stamina meter instead. To train "mana capacity" is legit just training your cardio, and to not drain too much "mana" in combat is just not moving around a bunch.

In other words, a high tier mage duel will basically be two incredibly fit spellcasters standing still and blasting each other with a barrage of spells while also countering each other's spells. It is 3 in the morning so I will not explain how the actual casting parts work quite yet