DULUTH — The half-dozen runners who had completed every Grandma’s Marathon since it started were known, in 1997, as “The Six Pack.” Within two years, they became the “The Iron Four.” The ever-dwindling gang had a decade-plus stretch as “The Iron Three” and a shorter stint as “The Iron Two.” Heading into the 50th Grandma’s Marathon on Saturday, just one of the original pack remains in the running. John Naslund, 76, of Bloomington, has crossed the finish line every year since the race began.