What's a good ab workout schedule for me?

I'm 13 and going into the 8th grade, and run Cross Country for my school. Our high school team won state last year and I want to follow in their footsteps. My coach (who coached the high school team last year also) had us do an ab workout before each run. I have already started long distance runs this summer and was wondering what a good ab workout would be for me so I can be in good shape before the next CC season starts. Thanks