Dorian was frank in his autobiography, mentioning his naturally high workout capacity. He was naturally strong and highly conditioned. If you are built that way naturally and juice to the gills, it is difficult for others to surpass your level of work.
I had seen a documentary on Rocky Marciano, who worked maniacally all year round. His success was significantly due to his obsessiveness and extreme competitiveness, but he was also built to last. You can't run 10-15 miles in heavy boots all year round, and regularly punch 300-pound heavy bags without destroying your tendons, unless you are built that way.
Most of the iconic Iron Age champions seem to have been blessed with a robust constitution. Those who weren't built to withstand such intense pressures and high workloads often ended up burning out.
Reg Park was another exceptional individual. He could eat 3-pound steaks without hardly chewing and was capable of working out with heavy weights even well into his 40s.
Genetics isn't a magical muscle structure that blow with some pills, it plays a significant role in an individual's capacity to work hard, take Arnold, he was relatively strong but his extreme aerobic capacity was the key for his superb conditioning, not many did 50 sets squats in their teens.
It is hard to say, whether he succeed because he worked hard, or he was built in a way to withstand such sick routine, this is common factor on other sports as well.
