Great job on the framing of the runners in this shot Peter, I always find it really hard with shots like this not to get an extraneous arm or leg somewhere. Very nice composition which defines the athleticism, concentration and hurt of marathn running (I did it once and NEVER again)
The lighting is a bit harsh and the background is cluttered, taking away contrast from the subjects. Perhaps by taking the photograph while the were passing through the shade of a trees or a bridge could have improved the lighting. As far as the clutter, narrowing the field to just one or two lone runners would make it easier to ensure a clear definition between the subjects and the background.