“We’re going to have to make the course harder,” Mary Ellen Adipietro, the Shelter Island 10K director, said with a laugh on Monday, two days after Simon Ndirangu topped a field of 1,066 finishers in 28 minutes and 37 seconds, setting a new course record. The 26-year-old Kenyan’s time this year was three seconds faster than Alene Reta’s winning time in 2010, when Reta, an Ethiopian, bested by one second his own 2007 record of 28:41.