To put things in perspective:
I was training back before you were born. I have also never been in a situation that required physical force. Just lucky? Maybe, but I believe it's more about alertness and awareness first.
Being "switched on" allows you to recognize and avoid a threat before it becomes a situation that you have to fight your way out of. Try this: next time when you're walking home, try "tracking" the people around you. Is there someone walking behind you? How many? How far away?
Budo is one way to develop this alertness. (Training in a crowded dojo keeps you aware of bodies being thrown in your general direction
There are also no real short-cuts to self defence. I can't remember how many times I have "died" doing knife defence.
However, my budo training has given me some confidence, and people can read that confidence. I remember one time when I did think I was going to have to fight my way out of a situation. I resigned myself to the idea of taking the "bad guy" down, and for some reason my body relaxed. They aggressor sensed this and backed away.
Most importantly, the question "How effective is aikido in self defence?" is far less important than "How effective is the aikidoka at self defence?
My ￥2 worth, and for free!