The one thing that can't be much fun for you is the long render times you are experiencing (I know you say you're only using one core, but even so). Has your computer only got one core, or are you only using one core for rendering by choice. You haven't got ray traced shadows enabled in the atmosphere or cloud layers, or soft shadows enabled? Seeing your render, lighting, atmosphere and cloud layer settings when you post your tgd may reveal why your renders are taking so long. Twenty plus hours for a render would dishearten me.
John