It's technique as much as training
Dr. Phil Maffetone’s Big Book of Endurance Training & Racing: Best principals of the sport and the easiest book to read (in my opinion).
TriCalc iPhone App: Great to calculate your splits for race goals
Training Peaks iPhone App: Review TP workouts and to log workouts
Triswimcoach Facebook: Friend triswimcoach on Facebook. He will give you good workouts and technique every week.
Total Immersion: The Revolutionary Way to Swim Better, Faster & Easier. Recovery periods in swimming, running and cycling intervals can be just as important as the actual interval itself.
Buy a Garmin 310XT or 910XT with a HR monitor
Run Off The Bike (ROTB) frequently even if it is just 1 to 2 miles.
Join a Masters Swim (MS) even if it is for a trial period and you go for a few times. This may make you feel uncomfortable, which is a good thing. This will help you.
Do strength training, even if you have to hire a strength coach.
Go into every workout with an objective; Endurance, Speed, Strength, Power or Recovery
No Static Stretching before workouts, especially speed, strength or force intervals. Ballistic stretching only.
Static & Ballistic Stretching after every swim, bike and run.
Eat protein and carbs asap after workouts. Real food is best but Hammer Recoverite works.
Incorporate Pilates into a weekly strength routine. It helps with stretching and strengthening the core muscle groups.
Be careful of junk miles
Listen to the body
Take 1 day a week completely off for recovery
When swimming, always remind yourself that nobody “runs out of air”. People actually “get too much of it” and they do not breathe it out. They choke on their own air (CO2).
Occasionally when doing endurance swims, put something at each end and look up to sight the object halfway down the length. Do this for the entire swim.
Set your watch to beep and notify you on the bike to take in nutrition.
Even if you are comfortable in aero, you may want to get a bike fit.