HR Zones and LTHR

At this point, don't think I'd change any training based on it. Just keep going with what you have been. You wouldn't over train from doing a test, but you could very well over think things by just starting use HR now. That includes using the Garmin calculated LTHR. No idea how it does that. By all means record the data if you like, but keep it more in the background.

I wouldn't worry about it either, keep on training and do the actual test after you recover form the big IM.

However, you could use the Olympic run as a sudo-test.  From what I remember for a run race 10k ~ 95% of the HR average would be close to a LTHR test result. My history for 10k run race HR average was just above my LTHR so it would give you an idea about where your LTHR may be, if you run the run portion all out.  If you don't run all out then I wouldn't worry about the result providing you with any real number.

FWIW, I found on a Garmin blog the following info on how G calculates HR zones from their test, also seems to be a lot of distrust in the test method, whatever it may be:

Maximum (Z5) - >100%
Threshold (Z4) - 95-100%
High Aerobic (Z3) - 89-95%
Low Aerobic (Z2) - 80-89%
Warm Up (Z1) - <80%

The BT LTHR percentages are:

Maximum (Z5) - >100%
Threshold (Z4) - 96-99.3%
High Aerobic (Z3) - 91.3-95.3%
Low Aerobic (Z2) - 85.3-90.7%
Warm Up (Z1) - 66-84.7%

These numbers are a bit diff and allow the HR zones to always have the next zone to be -1 & +1.

The Joel Friel/Training peaks % are:

Maximum (Z5) - >100%
Threshold (Z4) - 95-99%
High Aerobic (Z3) - 90-94%
Low Aerobic (Z2) - 85-89%
Warm Up (Z1) - <85%

When you are ready do the test as described here and use whatever LTHR zone calculation you wish.  I do see the 735XT allows you to manually enter HR zone percentages too if you don't want to use the defaults zones from Garmin.