Staying power is the ability to endure for an extended amount of time in challenging missions, events and mini events over the course of a long day.
Staying power or stamina (can be used interchangeably here) has a mental aspect, too. Fatigue can alter the sharpness required by tactical events. Therefore, physical and mental stamina should be sought. What we call the Triad here is physical, mental and attitude. Never letting one deteriorate.
It is absolutely necessary to train endurance and recovery. Short intense anaerobic bouts are good but aerobic endeavors are sought to recover here.