"It is true that the ones who come out on top are the ones who have been trained in the hardest school." - King Archidamus II of Sparta.
The Spartans were known for their discipline and harsh training regimen. These factors contributed heavily to their superiority on the battlefield. We're all familiar with the feat of the '300'. No one expects 300 soldiers to keep thousands of the enemy at bay for 3(2?) days without being both mentally and physically tough in the first place. The quote has been put in to emphasize the point: tough training makes a BIG difference (if not all the difference).
The average street fight lasts for just 30 seconds. But it's punishing on one's stamina and strength. Add the adrenaline rush, the fear of losing your life along with getting hit into the mix and those 30 seconds will feel like 30 minutes. This is where being 'tough' comes in. You overcome the fear and move in to finish the assailant without hesitancy. You don't bother too much about being hit because you can take the blows: you've been through it in training. And you're physically fit enough to go the distance.
Training tough means putting your body through more than what your mind wants you to take. It means never settling for less. Because that's what happens in a street attack: the odds are rarely in your favour and you don't get to decide the intensity of violence and when or how it ends. With tough training all you do is increase your chances of survival and gives you more control over the 'when' and 'how'(one shouldn't forget the importance of training realistically too).
The more pain you experience in your class, the tougher you get. It's a mindset that needs to be developed: you need to like the pain you go through since you know it's good for you. A real fight is dirty, ugly and brutal. The assailant is aggressive and has harmful intent. There's a risk of serious injury or even death. The one who cowers is sure to end up on the floor.
Before the class starts, you feel the fear (of what's to come); during it, you go through the pain....but after it, you come out a different person: quiet, assured (a little more confident than when you started the class) and a with sense of pride.....just like a Spartan.