simple fact is that everyone is going to die eventualy. maybe you'll be hit by a bus, maybe you'll wake up tomorrow with some disease that will kill you in 24 hours, maybe all kinds of things. You've known all your life that you were going to die though most of you probably think in terms of growing old first. However you HAVE known that.
Now, you can do two things with this knowledge:
You can accept it, not worry about it and live life to the fullest each day (since it might be your last).
You can pretend it'll never happen, you're immortal and live in secret fear knowing you really aren't.
Not sure how many fit into each category, but life is sure a lot more fun if lived from the first perspective.
I doubt many walk into the dojo with the realization that they might get killed during training, but the possiibilty is there. You could. You could land wrong and break your neck, you could have a heart attack, someone could land on you and do damage, anything can happen. Hopefuly wont but in any activity where you're flinging your body around at a solid object you can get injured (which is why you're supposed to be careful).
There's a huge difference between accepting the fact it could happen at any time, and obsessing on it however. Being ready to die doesnt' necessarily mean you are going to take active steps to see to it that it happens any time soon. It doesnt' mean you walk around with a gloomy face pronouncing doom on yourself and all around you every couple minutes.
It doesn't mean that if a mugger wants your wallet you dont give it to him. It does mean that if a mugger comes at you intending to kill you, you dont freak out that you might get killed, you deal with the situation calmly. You might die, however you're less likely to if you dont worry about whether you will or not while you're dealing with trying to make sure you dont than if you freeze up, panic and focus on that instead of what you should be thinking about at the time.
Long rambling post (shuts up now).