Escape from Famine in Ancient Times, the Male Protagonist and I Work on Infrastructure Construction

A postgraduate student in modern agriculture has been pregnant for fifteen years. He originally thought he was just writing about farming, but suddenly he was forced to run away from famine during a disaster. He met the grandson of the former dynasty who had hidden his identity, and led the refugees to build new cities and develop agriculture, but ultimately chose to return to modern times, leaving only a legend behind...