Heart Rate targets for HIM

Keep in mind that the percentage of LT that's optimum to race at will vary a good bit depending on how fast someone is and how much weekly training volume they've been doing.  Really fast athletes race at higher intensities, and slower athletes at lower intensities as a percentage of FTP or LT, because of the differences in time that they're on the course.  For someone who's biking in the 3 hour range, I'll typical hold their bike to 80%-85% of LT.  Harder than that, and most are risking blowing up on the run.  That's also assuming that you've established your LT fairly accurately.

For the run, we'll hold similar numbers for the first half, then switch over to RPE for the last half, building to red line for the last 3 miles.

When I did my first HIM it was unseasonably warm.  When I came off the bike my HR went right to high Z3 even though I was running at training pace with a comfortable RPE.  After 2 miles or so I was in Z4 for the duration at a pace were I was normally Z2.  Heat, dehydration, etc., will cause cardiac drift so be aware of that.  I ignore HR on the run during a tri race, RPE rules for me.  HR is for bike "control"!

Were you able to run off the bike?  If you were, it's not as hard as you could go.  Determining LT from an oly is a crap shoot, at best.  Better than nothing, but not by a lot.  If you ran well (whatever 'well' means for you), then you paced ok on the bike and you were probably somewhere in/around z3 for most of it.  

But, if you haven't trained with HR, I see no reason to race with it.  If your training was off perceived effort, then you should race that way.  Hopefully you've done some training at 'HIM effort' (or your guess of what that might be) and know how you felt during and after those workouts.  Those are your best gauge of how hard you can/should go on race day.  If this is your first, stay a little more conservative until the second half of the run.

Then you were probably well-paced in that race (though if comparable courses/fitness, you should not be able to hit your 10k PR off the bike if you pace well--I'll assume not comparable on some level).  I'd still be cautious about using your HR as a primary gauge unless you have some more data points to confirm what a reasonable target range is for yourself.  If you do trust your data and use your HR from the oly, then you would likely be around 160 using that chart above (though I would agree that is on the aggressive side for a first time HIM).