English already has the past simple for a finished story: I arrived. The past perfect is for when that story has another past inside it — something that was already true before the moment you are talking about.
I arrived at the gym. She had left.
I had trained for months before the race.
Two pasts, one earlier
| Earlier (past perfect) | Later (past simple) |
|---|---|
| She had left | I arrived |
| I had trained | the race started |
| The class had started | when I got there |
The past perfect does not travel well on its own. It wants that second past next to it, so the listener can hear the order.
Not a long time ago
A very common guess is that had means "ages ago." It does not. Five minutes is enough, if another past comes after it:
I had just sat down when the coach called.
The question is never "how long ago?" It is "which of these two pasts happened first?"