From the photograph you posted I see a couple of problems. The first one is a bit of blurriness due to camera shake. The 70-300 is not a fast lens so you may need to go to auto ISO or bump your static ISO to a higher number. A good monopod would also be a big help here.
The second thing I see is a bit of back focusing. The objects in the background are sharper than the subject, which I assume is the lead runner. This could be a lens problem or an operator error. And, it also goes back to the monopod suggestion... While the 70-300 is a slow lens, it's also a fairly heavy lens which can be difficult to hand hold... especially at slow shutter speeds.
To provide any further insight, we need the metadata Peter requested.
--Ron