As one who raced post concussion and learned the hard way, you don't have to skip the race. Just take it easy the week after, no killer gut wrenching workouts. Stay well hydrated during the race. You may feel crappy once the pace picks up. If you start feeling a little spacey and find yourself drifting back through the field, it might be best to DNF than risk passing out. Passing out in the field sucks.