Owen Anderson (who I believe is one of the most knowledgeable individuals on running) suggests increasing leg-muscle power during pre-marathon preparations, rather than by focusing on long, slow runs. I agree with Anderson and this was the key to how I trained and how I train people today.
 

Time after time I hear coaches describing a problem they see in an athlete’s biomechanics with no knowledge of being able to correct it because of a lack of knowledge of running mechanics. As such, they simply tell the athlete to focus or prescribe exercises or drills that only accentuate the problem.

Regardless of the level of runner learning proper running mechanics is important. Learning proper mechanics will improve your performance and help you avoid injuries.