When it comes to the fundamental of self defense there are two things you must keep doing, you have to keep attacking and you have to keep taking ground. It sounds simple, but it works, but just because the concept is simple it doesn't mean the execution is easy. In order to defend yourself you need techniques and tactics that will let you be as effective as possible in a potentially life threatening situation. If you're going to be ready you need to train, so when you're attacked you will know what to do without thinking.
No matter how careful we are in life there are times when we're more vulnerable than others and even the safest towns and cities have their crime victims. There are people who view moments like you standing alone on a train platform or walking through a poorly lit area as opportunities to strike. So if you can't run and help hasn't arrived yet you will have to fight and your strip mall karate or fitness workout martial arts won't cut it in a real street fight.
There are a lot of fun combat sports out there, but they won't necessarily prepare you for a real brawl. Take Brazilian Jiu-Jutsus for example which can be bloody and trains people to fight standing and on the ground it sounds good, but if your first move is to take the fight to the ground you're leaving yourself open to your attacker. Old school karate used to be all about killing and disabling your opponent, but mostly what is taught today is sport karate where many of the deadliest techniques have been removed. You might keep on attacking, but are you hurting your attacker or even putting yourself at a disadvantage.
Now if you're studying a real method of self defense it is just a simple matter of getting in the right mindset to use what you have learned in actual combat. Once again it sounds simple but if you have every been in any kind of combat you know it is a hell of a lot different than training, but you also know if you're training will carry you through a bad situation. So before you begin training find out if you're training in a style that is effective in real combat. Your training needs to be designed to hurt your opponent not score points.
You're confronted by someone who just won't leave you alone and you realize you have no where to run to and if you go for your cell phone you won't even have time to dial 911 before they're on top of you. So you strike before they can pull their knife, you might lead of with a kick, a chin jab, or an edge of hand. It doesn't really matter as long as you get out of their way and hit them where it hurts. You keep striking as you surge into them not giving them a chance to pull their weapon or even fight back. They might go down or you push them back far enough that you can make it to a exit and get away either way you did what you had to do. If you keep attacking and you keep taking ground you turn the tables on your aggressor and go from a victim to a survivor.