At which time a barricade is placed in front of the finish line, the clock turned off, and the finish chute deconstructed. Have a huge laughing clown there to heckle anyone who didn't make it.
The times:
20:00 5k
40:00 10k
1:30 half marathon
2:55 marathon
No other race distances should be conducted. Bananas at the finish line.
I reiterate: STRICT enforcement is a must.