2018 Half Iron?

I start at about 6-1/2 hours a week and build to about 9-1/2 hours a week on peaks weeks when training for he 70.3.  Some do a lot more than that some do less. My week looks something like two days of 45-60 minutes running, one day of 90-120 minute running, two days of 50-65 minutes cycling, one day of 120-180 minutes cycling, three days of 30-60 minutes swimming.  I try to do a interval, tempo, endurance day for each sport every week.  

I did Ironman St. George in 2016.  It was the most beautiful course I have seen.  The swim is breath taking, and when you are going up Snow Canyon 40 miles into the bike leg it is even prettier.  The course is difficult.  That is a plus for some and an negative for others. So it really depends on what you are looking for as to what race is the best. I want to do the Ironman Boulder next year.  It used to be put on in June, but was changed to August last year, so I am not sure if a hotter part of the year will make that race as nice as it would have been if I could have done it in 2016 before the switch in race dates.  There are 100's of 70.3 races.  I some like big races with more than 2000 people, some like races under 250 people.  Some like to ride through big cities with lots of spectators, others like to ride through pretty scenery, some like ocean swims, others like lake swims, etc.  Every race offers something different.  You can have a good experience anywhere you go.  

Of the three 70.3's that I have done St. George is the one that I would most like to go back to.  

My only HIM was 2006, and I trained minimally for that one.  

You'll want a long ride of 2-3 hours and a long run of 1.5-2 hours for your HIM.  You'll build to this and would expect these weeks to be a month before the race with a modest taper into the race.  You'll have a number of 45 minute workouts in addition to these long workouts, so add about 4.5 hours (6 workouts) to the 4-5 hours for your long workouts and you can reasonably stay under 10 hours per week even in peak weeks.  

As for race selection...Are you looking to "just race" or is the race an excuse to plan a trip somewhere?  There are advantages to both.  I did IMCdA and Vineman (both IM distance) in conjunction with 2-week trips, in the first it was a driving vacation that included Yellowstone, Glacier, & Mt. Rushmore, the latter, two weeks bouncing around NOCA including wine country, Yosimite, and SF.  I did B2B where we drove to Wilmington Friday, raced Saturday, and was back home Sunday.  

Moving from OLY to HIM:  Hydration/nutrition become much more significant (so get good practice on your long workouts).  Also, you'll be running during the heat of the day, so do some of your runs at that time as well.  (IMO, the noon-time run is what makes a HIM more brutal than even a full IM.)