Sit-ups and crunches don't do much for your running. What matters is whether your trunk and pelvis stay steady over a single leg, especially late in a run when you're tired.
Little and often is the key with these. A few minutes a few times a week is enough.
Free download
Core Control Exercises for Runners
The core control exercises I give runners, with the progressions. Enter your email and I'll send them over.
You'll also get my running tips by email. Unsubscribe any time with one click.