The event draws 3,500 riders for a variety of events. The signature ride is the 50-miles McDonald's Citizen Tour that is capped at 1,800 riders. It takes cyclists through an aggressive climb from Durango, at an elevation of 6,512 feet, to Silverton at about 9,300 feet. Along the way, riders must climb and descend Coal Bank and Molas passes, each exceeding an elevation of 10,000 feet.
Registration costs $95 and sells out quickly.
"I spent all day yesterday on planes, trains and automobiles to get here," said Ian Laxdal, 51, of Edmonton, Alberta, Canada. "I'm sure it'll be worth it, though. It's kind of a bucket list thing for me."
Farmington Mayor Tommy Roberts has ridden the Iron Horse three times, although this year he opted to visit family in Washington, D.C. Roberts said the ride has some attractive topography.
"I like climbing," he said. "You get a lot of climbing miles in that ride."
Riders said the Iron Horse is a notable challenge to complete.
"It's personal satisfaction," said Todd O'Toole, 49, of Durango, an Iron Horse regular. "I just race against myself. It's a great ride, really challenging.
The Iron Horse is the largest annual event for Durango's avid cycling community. Riders in training fill up spinning classes at local gyms for months beforehand, and when the weather improves, groups of spandex-clad cyclists fan out along the county roads surrounding the city.
While it's a question the Census Bureau fails to ask, it's safe to assume an unusually high number of Durango residents know their resting heart rate.
Part of the Iron Horse's attraction comes from roads closed to motorized traffic. State troopers shut down a stretch of U.S. Highway 550 from Durango Mountain Resort to Silverton to allow riders to proceed safely.
Farmington police were helping with the road closure and traffic control. The department sent four motorcycle officers to aid the event.
"They invite us up every year to come and assist," said Officer Marty Snowbarger.
Events today are highlighted by the criterium for pro-cyclists that loops around downtown Durango, and the fun-focused "cruiser crit," for which many cyclists don costumes.
Chuck Slothower can be reached at email@example.com; 505-564-4638. Follow him on Twitter @DTChuck