Welcome to the fold.
Before anything, you need to start building your base NOW. Sprinkle some interval work into your run since you're doing a sprint. But you're a runner, so I don't have to tell you this.
1- You need to find a race that takes place early in the season (2012) that way you can sign up for another if you really enjoyed your first. Keep an eye on the web page for the race and sign up ASAP, this will establish your commitment to the event.
2 - After you find this first race, come up with a workout plan to get you ready
3 - Once you have the workout plan, attack it full force and listen to your body, train smart and get your recovery in
4 - Sprinkle some brick workouts into the plan, experiment going from a swim to a run and experience what that is like, same with a bike/run workout.
5 - Adjust your diet accordingly.
A sprint is very achievable, but it depends on how you attack it. If you just want to complete it (which should be your goal, period) then you will be fine, as long as you have put in your work.
As far as the swim goes, most athletes distribute training times allocating the least amount of time to swim practice, which makes sense since this is the shortest of the three disciplines. However, depending how strong you are in the water you may want to put in more time in the pool. I did this last winter (3 X per week, 3000 meter workouts on average per session) and it got me a PR on my first event of the season it also gave me the confidence to start off from the front and inside.
Realistically, you should be applying yourself to the discipline in which you are weakest.