Training trial run to give confidence marathon pace is realistic?

I've raced two marathons so far, and both times I went out too fast and hit the wall. Reviewing the data, it appears I hit the wall ~14km after my average HR hits my LT2.

Marathon 1 - LT2 @ km 20, Wall at km 34

Marathon 2 - LT2 @ km 16, Wall at km 30

Prior to next marathon, I'd like to have a trial run to ensure my planned pace is achievable.

For example, 3-4 weeks before race, run 32km at goal marathon pace:

A) Never hit LT2 - Pace too conservative

B) Hit LT2 between km 27-32 - Pace correct

C) hit LT2 before km 27 - Pace too fast

Does this make sense? Or do any more experienced runners have their own version of a trial run?