Some readers are probably old enough to remember Jan Brady’s lament in “The Brady Bunch” about the seemingly undue attention that her older sister got: “Marcia, Marcia, Marcia!” If common road race distances were the Brady family, the marathon and the half marathon would be Marcia, getting all the attention. (“Oh, you’re a runner? Are you training for a marathon?”) The 5K (and maybe the mile, although that seems to be less common as a distance)…