WILLOW, Alaska — Defending champion Lance Mackey was among the early leaders in the 1,100-mile Iditarod Trail Sled Dog Race, hoping to repeat what once was considered impossible.

Last year, Mackey became the first musher to achieve back-to-back wins in the Iditarod and the 1,000-mile Yukon Quest International Sled Dog Race, believed by many to be an even tougher long-distance sled dog race. Fresh off his fourth consecutive Quest win, the Fairbanks musher is striving to achieve that feat again — without last year’s anxiety.

“People might expect me to do well here. As far at that goes, there is nobody putting pressure on me except for me,” the 37-year-old throat cancer survivor said Sunday as well-wishers flocked around him on the frozen Willow Lake just before the clock started ticking in the run to Nome.

Mackey and a record field of mushers set off Sunday afternoon on the competitive portion of the Iditarod.

Cim Smyth of Big Lake was the first musher out of the chute. Jessie Royer of Fairbanks and Jim Lanier of Chugiak were the second and third mushers out.

Mackey was fifth out, quickly jumping to the lead among 18 veteran mushers who agreed to carry a new tracking system that will let fans follow their progress online. Officials hope to expand the system to all participants in future races.

Mackey said he felt more prepared than ever for the 2008 Iditarod, compared to last year’s initial nervousness. That might explain why he was relaxed and joking two hours before the race to Nome started, while other top contenders were somber, even terse with onlookers.

Mackey said he considers mushing his line of work and if he doesn’t do it well, he will be forced to get a “real job” to support his wife and family. He’d rather keep mushing.

“I am pressuring myself basically to do well. If I don’t win this race again, I won’t be disappointed,” he said. “I can’t complain at all, not even a little bit.”

Six past winners and other top contenders are among a record field of 95 mushers, promising a highly competitive race over some of Alaska’s most unforgiving terrain. The world’s longest sled dog race kicked off Saturday with a short ceremonial run through Anchorage.

Rudi Niggemeier, a 51-year-old car dealer from Salzkotten, Germany, is among 33 rookies in the race. He’s run other races, but the longest was only 300 miles. After spending five winters in Alaska, however, running the Iditarod became a goal.

“Now I feel it is time to go to Nome,” he said. “I’m not a young man, but I’m only a little nervous.”

The Iditarod, begun in 1973, commemorates a run by sled dogs in 1925 to deliver lifesaving diphtheria serum to Nome.

The modern-day Iditarod trail crosses desolate tundra, thick forests and two mountain ranges along the frozen Yukon River, then goes along the dangerous sea ice up the Bering Sea shore to the finish line in Nome. Temperatures can plunge far below zero and winds can wipe out visibility for the teams.

Mushers are competing for a piece of an $875,000 purse, to be paid out among the top 30 finishers to reach Nome. The winner gets $69,000 and a new truck worth $45,000.