Re: keys. Do like the guys - have them in your hand before you head out into the parking lot. That way you won't have to stand by your car rummaging through your handbag looking for them.
Re: knee to the groin. If you do employ this time-honoured tactic, give it all you've got. My old judo instructor used to say, "Give him three Adam's apples." If you hit your assailant hard, it will disable him; if you just give him a little nudge, you will make him really mad.
And, you've heard it before but it can probably never be said too often: pay attention to your gut feeling. If you've got the feeling that something's not quite right, get out of the situation - you can figure out what it was that was not quite right when you're safe at home - or at the shelter, or wherever. Unfortunately, as is well-known, sometimes young women in particular are manipulated into remaining in dubious situations through their desire to appear sophisticated. In reality, no man loses his respect for a woman when she says, "I have to go now", if that man had any respect for her to start with.