From the upper quads to the shoulders and wrapping from the front of the body to the back, all the way deep inside your middle, lays your strength.
My last experience recovering from injury brought home the need for deep core strength. My hip strength was lacking and causing the rest of my body to make up for it. I noticed a remarkable difference from just one challenging hour-long Pilates class a week. Months into the new training I was on a treadmill and found that from the shoulders to my upper quads had become one strong unit like a pole from which my legs and arms moved.
Remember your strength comes from the middle of your body. Your legs work off of your core, with core stability transferring strength to your legs.
Using the winter months to develop your core strength will facilitate improved fitness in the spring. I am not speaking of sit-ups at all: zip, zero, nill. Sit ups will only strengthen a small percentage of the core. Plus, you most likely are using momentum to move and up and down and not truly developing the core. My favorite core routines are the main 18 Pilates exercises. I add in challenging moves with a large ball or foam roller, increasing my need for balance. I often use the book Quick Strength for Runners by Jeff Horowitz. It has 16 routines that I alternate between throughout the month.
Keeping the routine new and fresh causes a stronger core. Challenge yourself with balls, bars, and other equipment. Lay a bar across your back when performing donkey kicks. Add an inside shoulder width ball between your palms to your roll up. Balance on all fours upon a large stability ball. Anatomy for Runners by Jay Dicharry will help highlight ways to challenge your normal strengthening movements. Most of all become creative and playful with your movements.
A special note to women and especially to mothers with young children, it is vital to develop your core muscles. I remember being surprised during my first Pilates class that I even had muscles in my abdomen. I thought of the area as only being organs. However, those organs are encased in a wrapping of muscles that need to be identified and developed. You will find the difference in removing the stroller from the car trunk. If you struggle at all you need to prioritize core strengthening.
Develop your center strength to revitalize your running.