Running comeback now suffering.

Schmoo74

My brother in law asked me if I fancied going out for a run today. I used to run regularly but haven't been for ages. We went for a 5miler but by the end of it my legs were shot. Even though I stretch afterward I still knew my legs would ache a bit but I'm getting shooting pains on the outside of my knees. I have never had a pain in my knees before and I'm a little worried. Anyone have any ideas what it might be?