What is your core?
When I work with runners, I like to define the "core" as going from the middle of the quads through to the bottom of the rib cage. So we do exercises focused on the quads/hamstrings, the hip flexors, postural muscles and abs.
Specifically it's important to focus on muscles that aren't used when running. Since pretty much all of the running you're doing is in a straight line, it's important to incorporate exercises that involve moving laterally.
Side Plank w/ Leg Lift -- from the traditional side plank, raise your upper leg as high as possible
Why work on your core?
Good core strength doesn't necessarily make you faster on its own (although through better mechanics and a more efficient stride it certainly may), but it allows you to run more volume and at a higher intensity without the same risk of injury.
As I mentioned above, we generally run in a straight line, which limits the muscles that we use. Without supplemental core work, this can result in muscle imbalances that can eventually cause injury. By eliminating these imbalances, you can greatly reduce your chances of getting injured. Staying healthy means uninterrupted training, which ultimately translates to faster racing.
So it may not be news that working on your core is important for runners. But, knowing what muscles to target and what you're trying to accomplish can help you maximize your core routine and ultimately help you train faster and more often.
Do you have a favorite core exercise or routine? Include it in the comments section below!