If they couldn't even get over it then they wouldn't be of use to him in the future. He continued this practise for 2 hours straight without giving them any break.
He himself stood there and kept note of time while ordering them when some sort of small inconvenience happened.
After the 2 hours, he shouted out, "STOP! Its time to eat." He knew that their meals were incredibly important. He could see on their expressions, if they continued any longer, they would collapse.
The only reason they never complained wasn't because of their determination. It was because of fear. They feared making Saito angry and had thus continued to do as he said even though their fists had no real power behind them.
