Preheat your oven to 350°F (175°C). Grease and flour a 9x5-inch loaf pan or line it with parchment paper.
In a large mixing bowl, beat 1 cup of softened unsalted butter with 2 cups of granulated sugar on medium speed until light, fluffy, and pale, about 3 to 5 minutes.
Add the 4 large eggs one at a time, beating well after each addition. Add 1 teaspoon of vanilla extract with the last egg.
In a separate bowl, whisk together 3 cups all-purpose flour, 1 teaspoon baking powder, and 1/2 teaspoon salt.
Add dry ingredients to the butter mixture in three parts, alternating with 1 cup sour cream, starting and ending with dry ingredients. Mix on low speed just until combined.
Spoon half the batter into the prepared loaf pan. Dollop half of the peach preserves over the batter and gently spread it out.
Add the remaining batter on top, followed by the remaining peach preserves.
Use a butter knife or skewer to gently swirl the preserves through the batter to create a marbled effect. Do not over-swirl.
Scatter the 1/2 cup sliced fresh peaches evenly over the top of the batter.
Bake for 60 to 70 minutes. Check doneness with a toothpick; it should come out clean or with a few moist crumbs.
Cool the cake in the pan for about 15 minutes, then transfer to a wire rack to cool completely. Serve as desired.