Totem of the Great Wilderness

The protagonist is reborn into a tribe. Seeing how backward and weak it is, and how its people go hungry and cold, he uses his modern knowledge to find food, build a city, forge iron, fish, make pottery and salt, gather people from scattered settlements, and grow the tribe. Then he sets out to explore the mists and forests of the Great Wilderness. (This book has a heroine, but the protagonist isn't promiscuous. There's only one heroine.)