Good on you for stepping out of the box. Better than thinking I wish I had did that or why didn't I do that. Why not do it?
Join the Y or a swim team and start swimming laps. You can coast on the bike,and walk on the run but if you struggle on the swim the end result will not be good. So swim, swim, swim. In my experience swimming and biking are your main priorities. Still gotta run though. Curious, I have you completed a triathlon before?
Don't forget to have fun.