Fifty years after his historic Boston Marathon finish, Bob Hall still remembered the cheers near the Newton firehouse. Turning onto Commonwealth Avenue, heading east toward the city and one of the race’s most punishing stretches, he was greeted by a roaring crowd and the smell of sausages and cigars. Mr. Hall, the only wheelchair athlete in the race’s 2,365-person field, couldn’t help but crack a smile, even as he sweated his way uphill near Mile 18.