Mix sugar and water together in a pan and bring to a boil. Reduce heat to low and simmer for 1 hour or until the soft-ball stage is reached about 240 degrees on a candy thermometer. Add lemon juice, let simmer for a few seconds. Add walnuts, stir quickly to mix. Turn fudge out onto a greased cookie tray and press down the mixture to a round disk about 1 inch thick. While still warm cut into wedges or cubes. Serve warm or cold.