Australian 800m runner Kelly Hetherington gives a good insight into the type of core strength work she does as part of her training, and of it's importance.
http://www.therunningreview.com/2014/06/16/athlete-qa-kelly-hetherington/
So many runners at the sub elite level ignore core strength, citing it as not important. What do you think? Should all middle distance runners of all standards be treating core and weights as seriously as sessions and miles?