The feat -- unprecedented and, to some, unimaginable -- unleashed a torrent of accolades.

"Unreal," proclaimed Mackey.

"Unprecedented," said four-time champion Jeff King.

"The odds of winning the Iditarod and Yukon Quest back-to-back were astronomical," wrote the Daily News. "At one time, competing in both marathons was thought to be impossible. But winning both the same year?"

Added Mackey: "I guarantee nobody thought this was going to be possible."

Today, the adjective to describe the feat might be, well, routine.

Twelve months later, Mackey did it again.

And quickly, many mushers redefined what was possible. Although nobody has repeated Mackey's feat, there have been close calls.

In 2009, Sebastian Schnuelle won the Quest and finished second in the Iditarod. Last year, Hans Gatt did the same thing, first winning the Quest and then arriving in Nome about an hour behind Mackey.

In fact, five of the top six Yukon Quest finishers doubled with impressive results last year:

• Gatt -- First in Quest, second in Iditarod;

• Mackey -- Second in Quest, first in Iditarod;

• Hugh Neff -- Third in Quest, ninth in Iditarod;

• Ken Anderson -- Fifth in Quest, fourth in Iditarod;

• Sonny Lindner -- Sixth in Quest, 18th in Iditarod.

This year, seven mushers are entered in both races.

"Largely, we're finally realizing the capabilities of these dogs," said Iditarod chief veterinarian Stu Nelson. "They're adapting as they go down the trail.

"For years, people wanted to give dogs a long rest before a race, but it's better to put in long miles in the weeks before hitting the starting line. To be at that elite level, you need to be doing that kind of performance prior to the Iditarod in a race like the Quest. Or duplicate it in training.

"The point is, the dogs need to be working, putting substantial mileage on."

That advice flies in the face of what most human marathoners are told heading into a 26.2-mile race. Taper your mileage, is the usual advice. Keep your legs fresh.

Dogs are different.

A highly trained Iditarod dog would have a VO2 maximum -- a measure of the animal's ability to take in and use oxygen in the bloodstream -- of about 200 milliliters of oxygen per kilogram of body weight per minute. Lance Armstrong, who won all those Tour de France bicycle races, tested at an almost-unheard-of 85.

And dogs' muscles are full of mitochondria, those cellular energy-producing units. Some dogs have 70 percent more than humans.

But the way dogs process fat-rich diets may be the key. Several mushers have seen their dogs seemingly grow stronger as the race progressed, and racing back-to-back mushing marathons is just an extension of that trend.

Dr. Michael Davis, with the Center for Veterinary Health Sciences at Oklahoma State University, has discovered that fit dogs repair the damage caused by prolonged activity as the race progresses.

"You take dogs out and you run them 100 miles per day today and tomorrow and the next day, and they come back, sleep, eat, do it again without having any outward sign of it mattering," Davis told Outside magazine last year. "You would assume they're achieving homeostasis," a condition of optimal operation.

"You'd never think that they're at the furthest thing from homeostasis," he said. "They're damaging tissues, depleting energy stores, their oxidative stress is through the roof, and all those things are supposed to make you crater."

But they don't.

"What they showed us is that there is an ability to adapt to that stress in a matter of days so that it is no longer stressful," Davis told the magazine.

Not even when racing 2,000 miles in a month.

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Reach reporter Mike Campbell at mcampbell@adn.com or 257-4329.