Physical strength is built by pushing the limit to the maximum, breaking the micro-strands in muscle. The repair of the micro-strands builds the muscle, makes it stronger.
Mental strength is built by pushing the limit to the maximum. The experience of mental pushing is moving from comfort to discomfort. We learn the most when we leave the familiar to discover the unfamiliar, when we shift from the land of certainty to the land of uncertainty.
We still need time to repair. Mental repair is called integration. Mental repair is integrating the new experience from the land of uncertainty with things familiar that we know. Integration builds mental strength.
Pushing to the maximum requires risk and discipline. Sometimes the risk looms too large and discipline too hard. So, all we do is the minimum. And, if all we do is the minimum, pretty soon, our minimum becomes our maximum.