Maybe you're injured, or you're bored with your running. Perhaps you're looking for a new challenge. Whatever the case may be, making the jump from runner to triathlete is easier than you might think.
Since you're already accomplished at running, you'll want to focus more on getting comfortable in the swim and on the bike. Depending on your fitness, you should give yourself at least eight weeks to train for a sprint-distance triathlon (750-meter swim, 12-mile bike, 3.1-mile run) and 12 weeks to train for an Olympic-distance triathlon (1500-meter swim, 24-mile bike, 6.2-mile run).
You've already got one of the three disciplines mastered, so let's take a look at how to prepare for the swim and the bike legs of a triathlon.
The swim tends to be the biggest source of anxiety for those making the jump into triathlon. Learning how to swim properly (doggie paddle doesn't count!) will go a long way toward easing your fears.
Check to see if your local YMCA or gym offers swim lessons, or think about hiring a coach. Having someone on the pool deck to give pointers and feedback is critical to learning proper stroke mechanics.
Once you've gotten your swim form down, practice swimming in open water prior to race day if possible. Getting used to things like water temperature, wave conditions, navigation and wildlife can make race day a little less overwhelming.
If the water is cold enough for a wetsuit legal swim, you'll also want to practice swimming in your wetsuit before the event. You don't have to go out and buy a wetsuit, though. Ask around; Many triathletes have spare wetsuits and many bike/triathlon stores offer wetsuit rentals.