3x 200m, 400m, 600m. Rest the same amount it takes to run. 

Don't skip today just because its running.

I came across the following article about what CrossFit did for an avid runner. 

Summary: The reason why CrossFit improves runner's ability 1) It's a serious cross-training 2) It incorporates functional flexibility 3) It demands mental stamina 4) CrossFit is built-in high intensity interval training 5)Crossfit builds endurance