TALKEETNA, Alaska — Defending champion Lance Mackey is determined to claim a second win in the Iditarod Trail Sled Dog Race — and he's not wasting any time.

The 37-year-old Fairbanks musher was the first to reach the Takotna checkpoint, arriving at 1:57 a.m. Wednesday. He covered the 18-mile trip from McGrath in just under three hours.

Four-time winner Jeff King of Denali Park also checked in at Takotna, which is 419 miles into the 1,100-mile race, at 2:21 a.m. He followed six minutes later by Kjetil Backen of Porsbrunn, Norway, and Paul Gebhardt of Kasilof.

Also at Takotna were Jim Lanier of Chugiak and Aaron Burmeister of Nome. Five other mushers were en route to Takotna from McGrath.

Mackey, who last year became the first to record back-to-back wins in the 1,100-mile Yukon Quest International Sled Dog Race and Iditarod, rested his team for nearly five hours in Nikolai on Tuesday before getting back on the trail to Nome.

"I do the best I can with what I have in front of me," Mackey said last week about his race strategy. "I expect the worst and hope for the best."

Backen was the first musher to reach the checkpoint at Nikolai, checking in at 10:39 a.m. Tuesday with 15 dogs.

Backen coasted into the checkpoint, 401 miles into the 1,100-mile race. Backen told onlookers the going was rough on the frozen Kuskokwim River.

"I fall over but it's all right. No problem," said Backen, who was third out of Nikolai, 35 minutes behind King.

The leaders next will head west 25 miles to the ghost town of Ophir along an unplowed state highway, eventually dropping down into the Innoko River Valley.

Race organizers said high temperatures along the early stretch of the trail are the main concern in the Iditarod so far.

Race spokesman Chas St. George said some areas were reporting temperatures in the 30s, which "is too hot for the dogs to run in." A stretch between the checkpoint at Rainy Pass and Nikolai reported a high of 43, according to the National Weather Service.

St. George said last year about this time, temperatures were in the zero-degree range in this part of the trail.

At the same time, teams are dealing with heavy snow all along the trail.

"But there are no blizzard conditions," St. George said, "Everybody's moving."

About 20 dogs have been dropped so far, but there are no serious casualties, according to St. George. He said the numbers were tapering off.

"Usually in the first third of the race, mushers expect to drop a lot of dogs. This is an area for a lot of opportunities for sprains or other injuries."

Two mushers scratched Tuesday. Tom Roig of Shreve, Ohio, dropped out, citing concern for his health and the health of his team. Joe Garnie of Teller also quit, citing health issues with his dog team. A record field of 93 mushers remains in the running.

Until mushers begin taking a mandatory 24-hour layover and two 8-hour rests, the race is fluid.

In its 36th running, the race commemorates a run by sled dogs in 1925 to deliver lifesaving diphtheria serum to Nome.

The modern-day Iditarod trail crosses frozen rivers, dense woods and two mountain ranges, then goes along the dangerous sea ice up the Bering shore to the finish line under Nome's burled arch. Along the way, mushers can encounter temperatures far below zero, blinding wind and long stretches of frigid overflow.