My central story (essentially a quest and search for the truth about an old unsolved murder) featuring my main character and antagonist is rapidly coming to an end, but I've neglected to wrap up two sub-plots involving his revived affair with an old girlfriend and an old friend's burgeoning art career. I got carried away following the thread of the main part of the story. I don't know whether to carry on writing to wrap up loose ends in a sort of epilogue/final conclusion or to go back and do that about 7/8 of the way into the novel prior to the big ending.
Go back and tie it all in. Nobody really wants to read an extra 20 pages of sub-plot. Tie it all in so it's all neat and tidy and everything wraps up on your true ending.
Finish writing it and then go back and tie up the subplots. I've had to add scenes into my story several times now and I couldn't have finished the story if I had worried about them all along the way. You're almost finished, so just finish and edit in the subplot wrap-ups in your next draft. You'd be surprised what you can edit into a story.
