I try and drag myself up in the morning for a 'nice' long run/jog, and when I do, I don't have the energy to keep going...and my legs would feel all weak and shaky. Why is this?

You probably are weak from not having any food in your system. I know it is difficult for many people to eat in the morning but even if it was a high quality nutrition drink like a Myoplex Lite you should feel much more energy. Give it a shot!