The 22,834-foot Mount Aconcagua in Argentina one of the most accessible of the highest peaks in the Andes.

It was the quizzical looks of the children that finally alerted me to the fact that I had become ridiculous. On an idyllic Saturday, the weekend before Christmas, kids were sledding down a hillside in a Brooklyn park, making the most of a recent snowstorm. Their parents stood nearby, occasionally jumping in to adjust a scarf or tow a sled. And then there was me: a bearded man in tights and a bright orange hat, running up and down the same set of stairs for two hours, weighed down by a large backpack. One parent nodded toward me and said something to another. I glared at them: Yes, I know what I look like.

I'd been invited to climb Aconcagua in the Argentine Andes, the highest peak in the Western Hemisphere and the second tallest of the seven summits, that checklist of the highest points on each continent. My first thought: no way. I had less than a month to train for what normally requires at least two or three. I'm an active guy, prone to ill-advised adventures, but I'd never been above 14,300 feet. Aconcagua clocks in at 22,834. My second thought: Chance of a lifetime, no way you're turning this down, you better get to work.

The one thing I did have confidence in was the company I'd be keeping. The trip was being led by all-star guiding triumvirate Peter Whittaker, Ed Viesturs, and Dave Hahn, who had all been assembled by clothing company Eddie Bauer to help design a new line of technical gear called First Ascent.

Whittaker, 50, was born into the mountains. His uncle Jim was the first American to summit Everest, in 1963, and his father Lou, the co-founder of Rainier Mountaineering Inc., led several pioneering American expeditions. By the time Peter was 12, he was a guide-in-training. Viesturs, 49, is perhaps the greatest American high-altitude climber of all time and the only American to summit all 14 of the world's 8,000-meter peaks without supplemental oxygen. Hahn, 47, has climbed Everest 10 times, more than any other non-Sherpa, and over a 24-year career has gained a reputation as the consummate guide, equal parts workhorse, teacher, and sheepdog. He had never been to Aconcagua, and Viesturs had climbed it just once, 20 years earlier. (The mountain's reputation as an altitude training ground doesn't normally lure climbers of their caliber.) Whittaker was our Aconcagua expert, with eight ascents.

Initial focus-group testing of my plans yielded mixed results. "That's very high," said my doctor. "My advice would be not to go." My cousin Brian, an avid climber, gave me the unvarnished outlook. "It's a walk-up, pure and simple," he said, "but it's also a suffer-fest." This was in line with what I'd read elsewhere: that while certainly a challenging mountain with serious conditions, Aconcagua is technically undemanding, the sort of peak that a relatively inexperienced climber like me could conquer, with enough fitness, drive, and guidance. I was still haunted by unknowns – Would I be fit enough? How would I respond to the altitude? – but this was also part of the attraction, and I had long wanted to test myself with a trip like this. Ideally, I might have worked my way up from smaller mountains, but I liked the idea of bypassing such half-measures and starting big.

Training was a blur of early mornings, soggy running shoes, and chapped nipples, but by the end of it I had actually come to look forward to my routine. I was now flying up and down the 100 stairs on the north side of Fort Greene Park and had all but forgotten about the 30-pound backpack I was wearing. I was in if not the best shape of my life, something close. But I knew fitness would only take me so far and that mountains can be dangerously unpredictable places. Little did I know I was heading into what would turn out to be one of the deadliest seasons on Aconcagua.