There is a special tool called a Jones Counter which is attached to a bicycle and calibrated to a mile, or fraction of a mile by riding that accurately measured distance. Riding your racecourse and using the Jones counter on your bicycle, you can accurately measure your racecourse. I am sure there are many racecourses that are measured by driving them in a car and using the cars odometer, but this not generally admitted to. Most common distances are 5 kilometers or 3.1 miles, 10 k or 6.2 miles, 5 miles, 10 miles and a half marathon 13.1 miles and of course a marathon 26.2 miles. A popular course tool is the Gmaps Pedometer