Let us imagine a pilot, and assume that he had passed every examination with distinction, but that he had not as yet been at sea. Imagine him in a storm; he knows everything he ought to do, but he has not known before how terror grips the seafarer when the stars are lost in the blackness of night; he has not known the sense of impotence that comes when the pilot sees the wheel in his hand become a plaything for the waves; he has not known how the blood rushes to the head when one tries to make calculations at such a moment; in short, he has had no conception of the change that takes place in the knower when he has to apply his knowledge.~ Soren Kierkegaard ("The Storm" in "The Parables of Kierkegaard")
The first time I sparred in my Brazilian Jiu Jitsu class, it became immediately clear that there is a huge difference between the cleanness and neatness of applying a technique in class while you are learning it and when your partner isn't resisting much versus trying to apply a technique in the 'heat of battle' were everything seems to be happening at once and you are trying to defend against your opponent's attacks, counter your opponent's defenses and initiate attacks of your own. Out of a dozen or so 'fights' I 'won' two: one with an arm bar and the other with a figure-four lock (Americana) (which is my favorite lock in terms of level of pain.)
So what did I learn?
Firstly, I learned that the fitness required for a class and the fitness required for a fight are two different things; in a fight you tire quickly so it is important to both conserve energy and to end the fight as quickly as possible. The next day, delayed onset muscle soreness taught me that I needed to strengthen by inner thigh and latissimus muscles.
Secondly, I learned the importance of drilling technique until its correct performance becomes automatic; otherwise self-defence becomes just flailing about with all thought of effective technique flying out the window.
Thirdly, I learned that I needed to become more aware of what was happening in the moment, of openings my opponent was leaving that I could exploit, and of the imminence of a particular kind of attack that I would need to defend against to avoid being beaten.
Most of all, like the pilot in Kierkegaard's parable, I learned that all the theory you learn on land doesn't prepare you for the reality of being at sea.