Increasing physical stamina requires sustained focus and frequent effort, as opposed to occasional or intermittent bursts of intense activity. In like manner, if you and I are focused and consistent in developing our spiritual capacity, we can slowly and steadily strengthen our minds and hearts to discern, accept, and employ gospel truth; we can increase our spiritual capacity to detect and avoid temptation; and we can increase our determination and our ability to endure, through the enabling power of the Atonement, any challenges we encounter.
David A. Bednar, "Power to Become"
Physical stamina is necessary to endure physical struggles, whatever those may be. We hear of stories of others surviving a grueling ordeal and coming out on the other side. That, for certain, requires physical stamina in most cases. Along, of course, with a strong mental and emotional state.
Physical stamina does not just happen. It requires sometimes years of preparation and exercise.
Likewise, we need spiritual stamina to endure hardships and difficult ordeals as they hit us. And spiritual stamina does not just happen. It requires years of preparation and spiritual "exercise". Exercise that requires putting in the effort to read, to study, to learn and to know the word of God. It does not happen otherwise. It cannot.
So if we desire to survive spiritually, we need stamina. And we need hours of spiritual exercise to get it.