A half Ironman triathlon is a mix of speed and endurance.  triathletes need to be able to sustain fast and hard efforts over a 4-6 hour window of time.  Once you know you can complete the individual distances separately, your time is better spent focusing on your threshold intensity.  The higher you can raise your threshold power, the faster you can swim, bike and run on race day.  So much work can be accomplished in 2-3 hours on the bike.  A little speed and power will carry you far in your next half ironman or 70.3.  One distinguishing skill in half ironman racing is your ability to run well coming off of the bike.  You don’t have much time to slowly build into a good running pace.  You need to come off of the bike ready to get after the run. One workout that helps with this is the bike trainer/run interval workout.  Warm up on the bike and then complete 8 – 10 minutes at tempo and threshold intensity  followed by a  3-4 minutes transition run at your ideal race pace.  Athletes trying to qualify for the 70.3 World Championships, must earn a top age group spot.  Some half ironman races will also offer limited spots into the Kona World Championships.