Participation in America’s running races took a hit from the pandemic and its aftermath, but race officials say rebounding registration numbers for Colorado’s two largest road races, and others around the country, demonstrate the sport is thriving again. “It’s back,” said Andrea Dowdy, executive director of Denver Colfax Marathon, “and in a lot of cases, it seems like a resurgence.” Registration for the Colfax Marathon and its associated events — a half marathon, 10-miler, 5K and marathon relay races, all of which will be held this weekend — have surpassed 24,000.