In each of the past two seasons, the Mets’ improbable second-half runs began without fanfare. There was no signpost to tell the club it was navigating out of the abyss. No one knew for certain if it was happening at all. Perhaps Thursday’s 6-3 win at Marlins Park, New York’s fifth in six games, was another false start. Or perhaps it was something more. The Mets won’t know for months, despite their understanding that they must begin somewhere.