More than 23,000 of the men and women lined up in Hopkinton for the start of the 2019 Boston Marathon will be there having run under an age-specific time threshold in another marathon, which shows just how unremarkable qualifying for the Boston Marathon really is. And yet, if you ask any of the 500,000 Americans who will run a marathon this year (and many of the million-plus marathoners in other countries), qualifying for Boston - a ‘BQ’ - can be a bit of an obsession.