To answer this question, you need to know how to convert mile into kilometer. One mile is equal to 1.60934km. Then the runner speed in km/h should be: 6.8 mi/h * 1.60934km/mi= 10.94km/h.
Then the time needed should be:
time= distance / velocity= 12.3km / (10.94km/h)= 1.1243 hour *60 minute/hour= 67.45minutes