Right now I bet you're a runner semi-begrudging reading an article about triathlon. But you like chocolate milk, so you're interested, because you know I like chocolate milk too. I promise, I will make your next 3 minutes of reading worthwhile.
Triathlon is swim, bike & run. You're a runner so you've nailed the last part of the race, and often the hardest one to conquer. So, in all honesty, you're about 50% of the way there already. Now you just have to find a bike (almost any one will do - fixies included). We'll talk about the swim with a little more respect, as runner's often act and feel truly like a 'fish out of water'. I've been there. It's ok. You're not alone, in fact, I think you're in the majority.
Now, a triathlon. The first step is to pick one and sign up for it. I don't think it matters what distance, because you'll work backwards from your target distance. I would advise against picking an Ironman as your first one (although I wanted to). For me, a 1/2 Ironman was a good icebreaker. But then I think marathons are 'short' races now, so a grain of salt to you.
You have great aerobic talent from running, so you'll be able to translate that into biking - swimming...well, if it's your first time learning the sport, as it was for me, prepare to feel as unfit as you ever have. In about 6 weeks of thrashing around in the pool, things will start to improve. It will be a long 6 weeks, but after that, things just keep getting better and better.
That is, until you jump into the water for your first race with hundreds of other people. You'll laugh about that when it's all said and done (trust me) but you'll get out of the water, pedal that bike like the speed demon you are, and finally get to the most glorious sight of all: your running shoes.
You'll lace them up, feeling ready to rip right out of transition onto the run course, and you'll promptly start the most awkward running stride you've ever done. "What?!" "I have no legs!" "Help me!" - all things I've said out loud and/or have heard on occasion. But persevere. It gets better. Like any awkward teenager, you'll morph into the stunning young adult with about 5 minutes left in your race.
Then you'll cross the line! Your friends and family will be cheering. There will be way better snacks than what you'll find at the end of any running race. I personally go straight to the chocolate milk :) You'll be exhilarated with accomplishing your first triathlon - something you and I both know you were pretty nervous about. You've accomplished something that, at one point, you never thought was possible!
Taking on new challenges can be a life changer, take it from me.