Although Abbott led by 45 seconds heading into the final descent at Molas Pass, she already was losing ground to the hard-charging Durango local.
Asplund, a superior descender, kept bearing and bearing, drafting off a men's pro racer before she pumped furiously through downtown Silverton to the finish, but the 2007 defending champion was just far enough ahead - 8 seconds - that she never saw her challenger before the line.
"I didn't realize that she was so close until coming into Silverton, and by then it was a little too late," Asplund said. "Had I known that it was 10 seconds, I think I could've squeezed out a little bit more."
The 23-year-old Abbott, who rides with pro cyclist Mark Cavendish's High Point team and beat out Kristen Armstrong for the U.S. National Road Title in just her second year as a pro, bested her 2007 winning time by 1 minute, 18 seconds in 2:39:53.
Asplund also was runner-up to Abbott in 2007, albeit by a relatively robust 1 minute, 30 seconds.
"It stayed pretty interesting, because the gap kept going up and down," Abbott said. "Even though I was out in front the whole time, it was never a done deal until the finish. That made it really exciting."
Abbott opened up a lead of 40 seconds over Asplund, Alisha Welsh and Melissa McWhirter by the Glacier Club and expanded it to 50 ticks by Durango Mountain Resort at Purgatory.
McWhirter fell out of the trailing group, but Welsh and Asplund teamed up to narrow the gap to 25 seconds.
The race official notified Abbott of the charge, and the collegiate swimmer-turned-prodigious racer seemed to take it to heart, increasing the gap again to 1 minute, 10 seconds after the first ascent.
Asplund moved ahead of Welsh and was able to close back to within 45 seconds heading into the final descent.
Knowing Asplund has her number on the downhills, Abbott repeatedly looked back to see a closing Asplund but crossed the line with room to spare.
Her victory was no surprise to Mike Engleman, director and founder of the U.S. Women's Cycling Development Program.
"You watch an athlete early in their career and you know where they're going to go in the end, and you want to see them have spectacular days all the time," Engleman said. "I don't think Mara had a spectacular day, but it was good enough."
Abbott hails from Boulder and will head to Montreal next week for the World Cup, then race in Europe with her High Point team until August.
Asplund, 32, still was happy with her effort, if not her result.
"It was a great race," she said. "We definitely raced our bikes (Saturday), which was cool. Sometimes it's more of a hill climb, and it was a little more of a bike race (Saturday)."
Abbott said she expected a struggle with Asplund, even though Asplund had told Abbott she is focusing more heavily on the triathlon these days.
"Marisa lives around here, and she knows this race really well," Abbott said. "It's a home course for her. She's a good rider, and whether she's doing triathlon or doing road, she's somebody you have to watch out for."
Welsh, from Salt Lake City, only has raced for about a year and became a pro about a month ago when she took second in a stage to Kristin Armstrong in the Tour of the Gila, in which she finished 14th overall.
Viewed as a dark horse by Engleman, the 26-year-old crossed in 2:40:22 and raised some eyebrows along the way.
"About two weeks ago, somebody said 'Hey, you should look at the Iron Horse,' so I looked at the profile, and I said, 'That looks like an awesome profile,'" Welsh said. "I decided two weeks ago to come over and do it, and I loved it.
"I'm hoping there's some more fun in the future."
McWhirter, from Scottsdale, Ariz., crossed 2 minutes, 11 seconds back of the lead trio in 2:42:33.
Boulder's Amy Dombroski took fifth in 2:43:48.
Engleman, an 11-year-pro for Coors Light, U.S. Postal and Navigators teams, said a win at the Iron Horse carries great weight in the world of women's pro cycling.
"It's one of those races that doesn't always fit in everybody's schedule, but it's a race that everybody wants to win, particularly the climbers," he said.
mpiper@durangoherald.com