Being a runner myself, I will tell you that strength has little to nothing to do with running. I am quite weak physically, and I can easily outrun my muscular taller friend. I can outrun him and out endure him, as I have focused on cardio.
Having been a runner for many, many years in the Marine Corps, I can tell you it is all about Strength and Endurance (Body Conditioning). However, just becasue your legs are strong does not mean that you are considered overmuscled. At my prime, I could leg press 1400 lbs (Single Rep, Max Weight), and worked out with just under 1000 lbs. During that same time, I could not bench press my body weight (Single Rep, Max Weight, 130lbs - My weight was 174 lbs). In the Gulf, my combat load weighed almost as much as I did.
Yes, Agility can figure into it too a very small degree, but Strength and Body is where it is for running and movement.
