Jason Hartmann looks like an unlikely marathoner. In Monday’s wilting mid-80s temperatures, the 6-foot-3-inch, 31-year-old Michigan native came striding down Boylston Street, his pace strong, his head held high, still boasting enough energy to wave to the crowds. He ran across the line to claim fourth place in the Boston Marathon in 2 hours, 14 minutes, 31 seconds, the first American to finish.