BACK THEN is very remote, like school days or childhood.

''в тот момент'' in Russian can be used to describe something from long ago and something from recent time.

But “tired” normally doesn’t normally span eras… this probably refers to yesterday or last week.

Why not?

I vaguely remember my first marathon. It was in high school. I barely finished it. I don't even remember my time because в тот момент я был слишком уставшим to look at the display/screen. I just fell to the ground.

Yes, you were tired AT THAT MOMENT, but not DURING THAT PHASE OF YOUR LIFE.

So, you're saying we can't use BACK THEN describing a single moment in time? Is BACK THEN always something that stretches in time? In other words, is it always a time span?

Yes

Especially if it contrasts with the present: Back then, I was a smoker. (=now, I don’t smoke)

ok, I've edited it. looks good?

yes

thanks.
so basically, when we use BACK THEN we would usually describe a continuous action in file?

or one period or phase of your life

or of the world/life in general: back then, there was no internet.

👍