Pour the lemonade concentrate into a large pitcher or cooler with a spout. Stir in enough water to make 1 gallon. Grate the zest from the lemon and add to the pitcher, then squeeze the lemon juice from the lemon into the pitcher. In a small bowl, crush the mint leaves into the sugar using a muddler or a wooden spoon. Stir into the lemonade. Cover and chill for several hours, then strain to remove the leaves and zest before serving. Serve over ice and garnish with a sprig of mint.