For me running is medicine for anxiety.
Im not a medical person but the way I understand it, it makes the brain release all these chemicals, the brain gives us the drugs we are supposed to have. The brain and body have evolved to handle physical stress. I think when the brain doesn't get physical stress it creates its own mental stress.
I trust in running like I trusted in alcohol. I know once i start putting one foot in front of the other I will feel better.