Combine the graham cracker crumbs, melted butter, and 1/2 cup sugar in a mixing bowl. Mix until evenly moistened, reserve 1/4 cup of the mixture for the topping. Press the remaining mixture into the bottom of a 9x13-inch baking dish.
Heat marshmallows and milk in a large saucepan over low heat and stir until the marshmallows are completely melted. Remove from heat and cool.
Whip cream in a large bowl until soft peaks form. Beat in 1/3 cup sugar until the cream forms stiff peaks. Fold the whipped cream into the cooled marshmallow mixture.
Spread 1/2 the cream mixture over the crust, arrange the peaches on top of the cream, then spread the remaining cream mixture over the peaches. Sprinkle the reserved crumb mixture over the cream. Refrigerate until serving.