They say when the SHTF, and its for real, you can expect to lose at least 50% of your skill that you have in a relaxed situation (for example, on the range, when you think you don't need more training).
When you do take a class, and they start working in things like movement, speed reloads, tactical reloads, shooting from cover, issuing verbal commands, and responding to unexpected situations, you will learn that you need more training.
If you've never shot competitively, IDPA or similar, that is a great way to find out how much you need training. Getting those sights lined up when the pressure's on is a lot harder than you'd think.
Basically, if you haven't had any training, you suck, and you need training if you want to be at all proficient in any kind of real world situation.