Transmigrated as an Old Emperor, I Get Stronger by Taking Concubines

After a nap, I transmigrated. But I became an old emperor who would die the next day. Foreign enemies invaded, and my good son fled south. No soldiers, no food, no silver. Starting reward: an inexhaustible kidney. From then on, only fields could be worn out, not the ox. Spreading branches and leaves, getting stronger by taking concubines. How did the old foolish emperor become a great eternal ruler?