Just over a half-century ago, a mile-long oval began to rise on 200 acres of farmland on the east side of DuPont Highway in Dover. Unlike the epic film “Field of Dreams,” it was hardly a Delaware example of “if you build it, we will come.” “We had 22,000 seats and we struggled to fill them,” recalls Denis McGlynn, hired in 1972 to handle publicity for Dover Downs, then a 3-yearold horse and auto racing track, and now president and CEO of Dover Motorsports Inc., celebrating its golden anniversary this year.