Here's a core workout I did with the Columbus Running Company training group. It hits every aspect of your core--abs, obliques, spinal extensors, hips--which will keep your running form stronger for longer during that race! This is safe and effective to do every day, but try to at least get this series in 2-3 times per week. You'll be glad you did on race day! BUT even if you don't run, it's still a nice little series of exercises!
Start in a plank from straight arms:
1. Twist upper body through waistline, lifting your arm to ceiling, 10-12 times per side
2. Twist to full side plank and hold, 20-30 seconds per side
3. Lower to elbows and twist lower body through waistline, tapping your hip down to the floor, 10-12 times per side
Back extension lying on stomach, 20 times
Flip onto side with legs straight out (in line with hips. Make sure upper body stays as still as possible throughout.) Do 20 of each:
1. Lift top leg up toward ceiling and lower back down
2. Kick top leg forward and backward
3. Hold leg at forward angle and lift and lower leg
Repeat on other side.