Here's the thing to keep in mind: Self Defense classes are not going to make you able to fight. The best thing that they will do is show you what to look out for to prevent yourself from coming to harm (staying away from alleys, if you think someone is following you, walk into a group of people, etc), and some best practices/things to keep on you to minimalize any fighting (mace in your purse, rape whistle, running into a crowd and screaming, etc).
The only way to really be able to defend yourself in a physical sense is taking repeated lessons in martial arts. Just because a guy shows you once how to roll somebody over your shoulder, it doesn't mean that you've now got the instincts and muscle memory to do so without hesitation when it comes to a real world situation.