I'm thinking specifically of two classic races, San Silvestre de Vallecana and the Corrida Internacional de São Silvestre, but those are far from the only such races. What is it about this saint that gets road races where others don't?
Is it as as simple as New Year's Eve being the saint's feast day and also a convenient day for road racing (in the same way that Thanksgiving is a big day for road racing in the USA)? Or is there something particular to associate the saint with running?