Seneca's rehearsal
Seneca advised practicing poverty for a few days each month — coarse food, hard bed — and asking, 'Is this what I feared?' Hardship chosen in advance removes the fear of hardship imposed later.
The physical parallel
Musashi trained with a wooden sword against live blades. Modern equivalents: cold water, heavy loads, fasting, hard conversations. The point is not suffering; it is proving to your nervous system that discomfort is survivable.
The failure mode
Hardship as performance is vanity. If nobody would ever know and you'd still do it, it's training. If you need to post it, it's theater.