What’s the planet’s most legendary marathon? Probably that fabled first one Pheidippides ran in 490 B.C., breathlessly proclaiming a great military victory before perishing from his effort.Next in line? Surely the Boston Marathon, whose colorful 121-year history is captured in a movie premiering Saturday in its host city.Narrated by Matt Damon, “Boston,” the first feature-length documentary about the race, tells how it grew from 15 runners in 1897 to become the globe’s most venerable footrace.The world premiere at the Boch …