That’s a good question. First, your AeT for running will likely be different from your AeT for hiking. So that’s the first thing to take into account when choosing the HR to start at.
Do you already have some data from hikes or runs using a chest strap monitor to record your HR? If you do, and you even casually monitor your HR during the outing, you likely have a sense of the point at which the workout starts to feel hard — once it goes beyond the “all-day” pace. That would be the HR to start with.
It does take a bit of trial and error, but the good news is that you only have to play this guessing game the first time. Once you’ve established your AeT you then have a baseline.
Does that help?