I made a similar switch to what you're doing, learned to swim mid-high school and picked up triathlon. I was running mid-17s at the time but constantly injured. I was always good in the weight room, even as a runner, with a previous background in wrestling, so I had the strength to swim well and just needed to learn the technique. My recommendations to you:
-Start by putting the majority of your time into learning to swim properly. If your high school has a team, try out. It doesn't matter how bad you are currently. Racing the 50 and the 100 free and daily competition in practice will get you way beyond where you would get on your own. Having a coach on deck making corrections to your stroke is critical early on, you don't want bad habits.
-Start out with sprint distance racing. Master the fundamentals - getting out hard on the swim and getting into a fast pack, biking hard (but not cooking yourself), then running strong off the bike. Bike for show, run for dough. If your swim comes along well, try draft legal racing - it is the absolute best competition available to junior racers and will put you at a different level as you move up in distance.
-Learn to bike hard. A lot of runners do not understand what this truly means. Learn to apply a lot of power to the pedals for an extended period of time. I thought just because I was breathing hard and sweating a lot, I was biking "hard". Get a HR monitor or a power meter and learn to push yourself on the bike.
-Don't forgo training the run just because you have a background in it. The amount of cross training I did ultimately made me a better runner, because I was finally able to stay healthy. I went from running mid-17s in HS XC to running mid-17s off the bike. Get in a long run and a tempo each week.
-I followed this training split for years and continued to improve, work up to it then wash, rinse, repeat:
M - AM 75min swim practice (long intervals/pulling), PM lift
T - AM 75-90mins bike (2x20min tempo or similar - 40-50mins of work at 85-95%), PM 30-40min easy run + strides
W - AM 75min swim practice (sprints - hard 25s/50s/75s), PM 45-60min run w/tempo
TH - AM 75-90mins bike (5x5min, 10x3min, etc - VO2 max work), PM 30-40min easy run + strides
F - AM 75-90min swim practice (threshold - 20x100 - race pace repeats), PM lift
S - AM 2hr bike (did a lot of tempo in the last 30-60mins), PM 45-60min swim practice (pulling)
S - AM 30-45mins recovery ride right into a 75min run (maybe easy, sometimes a progression)
I started out with triathlon at the beginning of my junior year of high school and worked up to this split by fall of my senior year. Used this, or slight variations of it, for about 4.5 years straight.