The best stories are slices of time from fully-formed worlds. Dramatized, for sure. Larger than life, of course. But time goes on, both before and after the series of events that make up the particular hero's journey. There's no real beginning, and no true end.

Weaving details of the past and future into a novel is an interesting challenge. If your narrator is close enough to the characters to get inside their heads, thoughts and memories are hugely useful, as long as you make sure to avoid too much telling (vs. showing). Conversations, especially with new characters unaware of past situation, are another great tool (though infodumps are a common trap). Sidetexts can be a fun way to flesh out your world's history, as long as you don't rely too heavily on them in the main story.