How do I stop the early morning stiffness and pain radiating down one leg?

Exercise. Am pain upon arising is usually just related to some degenerative changes that cause some muscular tightness that usually resolves once up & about but may be an early warning of things to come if not proactive in terms of regular exercise including stretching, core strengthening and cardiovascular routines. Weight control is important as is cessation of smoking if you have this habit.
See your doctor. Your symptoms could have a variety of causes, ranging from arthritis to nerve compression to a slipped disc in your back. Please see your doctor to determine if any testing or treatment is needed. I hope you feel better soon!