Can you suggest a proverb that would bring out the moral of the story "After Twenty Years"? Discuss it in brief.

A proverb that comes to mind is from Aesop's fable "The Tortoise and the Hare":

Slow and steady wins the race.

There is nothing flashy, quick, or flamboyant about Jimmy. His friend Bob says of him,

He was a kind of plodder . . . good fellow as he was.

Jimmy never leaves his hometown—twenty years after Bob has left to make his fortune, Jimmy is still living in New York. All these years, he has been working faithfully as a police officer. Bob, meanwhile, has pursued a criminal path out west and arrives back in New York for their planned twenty-year meeting sporting a diamond scarf pin and a diamond-studded watch. On the surface, he seems to be the winner. He seems to have raced ahead of his old friend.
Nevertheless, Bob notes that Jimmy is steadfast, saying,

I know Jimmy will meet me here if he's alive, for he always was the truest, stanchest old chap in the world.

Jimmy is like the tortoise, moving ahead slowly and steadily toward his goal of living an honorable life. He arguably "wins" in the end, arresting his old friend and showing that steadfast purpose counts.