Heat oven to 350° F. Grease two 1 1/2-quart loaf pans. In a small bowl, combine half the sugar, the melted butter, and 2 1/2 cups of the walnuts to make the topping. Divide the mixture evenly between the pans, covering the bottoms evenly.
To make the filling, cream the remaining sugar and the softened butter in a medium bowl. Stir in the flour and the remaining walnuts. Set the mixture aside.
In a large mixing bowl, combine the cake mix, eggs, and oil, along with the amount of water called for by the package directions. Beat until well combined. Pour 1/4 of the batter into each pan, over the topping. Spoon the filling mixture evenly over the cake batter, dividing it between the 2 pans. Add the remaining batter, dividing it evenly between the loaf pans. Place the loaves in oven and bake until a knife inserted into the center of the cake comes out clean, about 40 minutes. Let cool 10 minutes, then unmold bottom-side up onto a cake plate. Serve warm or at room temperature.