Videos on social media showed Son Heung-min and Harry Kane falling to the ground after training drills at the Seoul World Cup Stadium on Monday, before Spurs' 6-3 friendly win over K League XI during their pre-season tour of South Korea.
"It's not like anything I've done before. This is hard, it's the hardest pre-season I've had," Doherty told British media. "I don't know if there will be any teams fitter than us.
"You get great satisfaction once the session is over. Your mind has been tortured in the middle of it but when it's done you feel proud.
"You're just thinking, 'don't quit', even if you crawl over the line or have to jog or walk towards the end. It's that determination the manager has instilled in us to keep going and never quit."
Spurs will face Sevilla in a friendly at the Suwon World Cup Stadium on Saturday.
