In his encounter with Crablante before he started training, Saitama is hit by him and thrown away like 30 meters or more, and then hit again 2 more times with those gigantic claws Crablante has. If Saitama was a regular human, how could he support this beating and still stand like it was nothing and fight back?
Seems like Saitama doesn't really know he has it before then. From what I remember, Genos said when he was train fighting with Saitama that he wanted to show or let know what Saitama's power really is, something like that.
If Saitama was able to even attempt to do his physical training regimen of 100 pushups/ 100 situps and 10km run, he had to be physically capable to a certain extent to even attempt something as vigorous. So even at that time, he might've been as strong enough to be a high ranked C class hero or low ranked B class hero perhaps. Also, Crablante is only a Tiger threat level monster.