Preheat the oven to 325°F (165°C) and gather all your ingredients in separate bowls.
Grease and flour a 10-inch Bundt pan, then evenly sprinkle the chopped walnuts over the bottom.
In a large bowl, combine the cake mix and pudding mix.
Add eggs, ½ cup of rum, ½ cup of water, and oil.
Fold in the chopped pecans and pour the batter gently over the chopped nuts.
Bake the cake for about 1 hour, or until a toothpick inserted into the center comes out clean.
While the cake bakes, prepare the glaze by combining sugar, butter, and ¼ cup water in a saucepan over medium heat.
Bring the mixture to a boil, stirring constantly, and cook until slightly thickened and darkened—around 5 minutes.
Remove from heat and stir in the remaining ½ cup of rum.
Once the cake is done baking, let it rest in the pan for 10 minutes.
Invert it onto a serving plate and brush the glaze generously over the top and sides.
Serve slices warm or at room temperature.