Annabelle Williams NEW YORK—One island. 32 miles. 3,500 masochists. The first Saturday in May is a cherished tradition for New York's die-hard walkers, who gather to circumnavigate Manhattan in one very long day. But lately the event has been almost unrecognizable to veterans of the 40-year-old stroll. A surge of interest from young people has made just gaining entry a chore. As recently as 2013, fewer than 1,000 people registered for the event.