I was re-reading some old posts (because I'm a perfectionist) and I realized I left out an important point.
Just because you swam in high school or spend every weekend on a bike or run marathons as a hobby doesn't mean that that's your strongest sport. I know a couple of people who have come into triathlon from cycling or swimming only to discover that they're excellent runners. So my last piece of advice when looking for your strengths is to keep an open mind. Who knows? Maybe you'll discover a new talent. And if you allow yourself to grow in new directions, your performance can only get stronger.