How did Saitama get strong in One-Punch Man?
One hundred push-ups, one hundred sit-ups, one hundred squats, and a 10 km run. Every single day.
That is the whole answer Saitama gives, in volume 2 of the manga, in the chapter Viz titled “The Secret to Strength.” Three years before the story opens, Saitama was an unemployed job-hunter who stepped between a monster called Crablante and a boy it was hunting. He won, barely, and started training that day.
THE REGIMEN, AS STATED
Volume 2, in his own wordsThe daily set
100 push-ups. 100 sit-ups. 100 squats. A 10 km run. Every day, without exception, plus a proper diet: three meals, a banana in the morning will do.
The hardship clause
No air conditioning in summer and no heating in winter, to build mental strength. Saitama presents this as part of the training, not a side effect of being broke.
The cost
The training broke him in the literal sense the story intends: his hair fell out within a year and a half, and his emotions flattened. The power and the emptiness arrive together.
The limiter
The series later gives the joke a mechanism. Meeting Saitama in volume 18, Dr. Genus concludes that he removed his limiter: the ceiling every living thing is born with. No character in the series has matched it since.