I started them last year (sprints). I've signed up for Olympics this year, one of which includes the swim in a river. THAT will be challenging.
1. Balance your workout with run, swim, and bike, but if you have a weakness, work more on it (mine is swimming I think).
2. Work in some brick workouts where you transition from swim to bike or bike to run. That bike/run transition is funny feeling on the legs. Fast paced 3-5 mile ride, then run a mile, then back to bike...
3. Train with friends if possible. Our area has triathlon training groups for beginners. That's what my wife and I did as we prepped for the sprints.
4. Consider finding some duathlons in the area (usually run/bike/run) to get some practice in.
5. For swim and run, consider speed workouts to increase your overall speed, though the speed sections may be shorter (such as 50m sprint in pool or race pace + for an 800m section). repeat.
6. Focus on good nutrition to get the nutrients you need. With the endurance stuff, I found myself getting iron deficient, so I upped the spinach and red meat consumption (along with more supplements).
I do not use any protein shake stuff or any of that powder stuff from places like GNC. Natural foods and supplement (pills) only for me.
As jeepTB mentioned, your tri length will really dictate how you train. If the triathlon uses a pool, get pool time in. If it is open water, get open water time in. If you will need a wet suit for the swim, get it now and get practicing with it, but not every swim session. If you can't get to open water for training, go to a pool while they're doing a water aerobics class. The water will churn pretty good and give you a more realistic feel for open water.
Above all, do it and HAVE FUN with it!!!! Don't stress about getting on the podium for your age group. You'll come up against a lot of folks in your age group that have things down and FLY. I don't stress about my swim speed. Going relaxed, but efficient pace/form, is better than going balls to the wall furious only to finish with maybe 20 seconds difference. If that is the case, practice transitions from swim to bike and bike to run. I have bike shoes and running shoes (clips on bike). I use speed laces that I don't have to tie. Little tricks like that are where you can really gain some good time with a bit of practice.
You won't lose it on the swim timewise, but you can certainly help yourself in the transition area by laying out gear logically and ready to use. I even put a bit of babypowder in rolled up socks so I can go from swim to bike without worrying about socks sticking to wet feet... little tips like that will help too.
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