Marc D. Witkes had mastered the Tucson Marathon several times and set out Sunday to tackle the race again.
But the Durango, Colo., man collapsed less than a quarter-mile from the finish line and was later pronounced dead, officials said.
This was the first time that a runner has died during the Tucson Marathon since the event began 13 years ago, said marathon event manager Marilyn Hall.
Witkes, 40, broke down at 10:51 a.m. near North Oracle Road and East El Conquistador Way, said Elizabeth Wright, an Oro Valley Police Department spokeswoman.
"He froze and collapsed," said co-runner Marjorie Brinton, whose husband watched Witkes fall.
Witkes was in cardiac arrest when paramedics arrived, said Anne-Marie Sweeney, a Rural/Metro Fire Department spokeswoman.
Rescuers tried to resuscitate him, and he was taken to Northwest Medical Center Oro Valley, where he was pronounced dead, Sweeney said.
"We are very sorry for his family," Hall said. "We understand he was very highly regarded by his family and running club. The whole running community is mourning the loss."
Witkes came to Tucson with a group of friends from Durango Motorless Transit, the Durango running club.
Some, including Witkes, planned to qualify for the Boston Marathon, which will take place in April, said Brinton, the Motorless Transit president.
At 21 1/2 miles, Witkes had run at a pace of 6 minutes, 35 seconds per mile, according to the race results.
"He felt great this weekend," said Brinton, who started the race with Witkes. "He was so upbeat and positive."
The ultramarathon runner had a sports blog and a Web site, on which he wrote about his passions: running, the Red Sox, biking, Durango and Colorado mountain living.
An athlete for 20 years, Witkes had competed in 30 marathons and 25 ultramarathons, including the Double and Triple Ironman and the Sri Chinmoy 700-mile run, he wrote on the Web site.
Witkes also freelanced and wrote columns on running and cycling for the Durango Herald newspaper once a month.
"He was very well-known in the running community in Durango," said David Buck, interim managing editor of the Durango Herald. "One thing Marc was good at was telling stories about unique people in town. He was so enthusiastic."
Witkes does not have family members who live in Durango, Brinton said.
He was president of Motorless Transit from 2000 until 2006. During that time, the membership skyrocketed to more than 260 members, she said.
"I attribute a lot of this to Marc," she said. "He really put his heart and soul into it."
It's relatively common for runners to have minor complaints such as a twisted ankle or dehydration, Hall said. But serious injuries rarely happen.
There are aid stations with water every two miles along the course. There also is a medical tent at the finish line, Hall said.
"We run a very safe event," she said.
Brinton, who also runs marathons regularly, said there could be several factors that might lead to a collapse, even if a runner has a healthful lifestyle.
"There are a lot of things that can go wrong when you're exerting yourself," she said. "You're going 100 percent."
According to The New York Times, at least six runners have died of heart problems during a marathon in 2006. Three of those runners were in their early 40s.
This comes after years in which almost no deaths were reported.
The Times cites two studies — by the Massachusetts General Hospital and the University of Duisburg-Essen in Germany — that suggest marathon racing can hurt the heart under some circumstances.
But no definitive results have been published, the Times said.
Witkes' cause of death was unknown, pending the outcome of an autopsy.
"We're feeling pretty shellshocked right now," Brinton said. "He was an incredible person who loved life and loved his sport."