“In our sport, you don’t become the best at age 18 or 20,” Mr. House said. “You become the best in your 30s and 40s, because it takes that long to get good at all the components that go into it.”

Mr. Steck’s work ethic “made him capable of accomplishing things that were simply otherworldly to the rest of the climbing community,” Mr. House added. “What he did was nothing short of miraculous, not only the speed records, but what he did and how he did it in the Himalaya. People just couldn’t understand it.”

Mr. Steck, who was born in Emmental, Switzerland, was known for his determination to complete challenging climbs, despite evident risks. A 2012 profile in Outside Magazine recounts how, after pledging to his wife, Nicole, that he would not make any more solo ascents, Mr. Steck endeavored in 2011 to climb Shishapangma, in Tibet, with a partner, the climber Don Bowie. After Mr. Bowie found himself unable to continue, Mr. Steck raced up the rest of the way alone. On Sunday, Mr. Bowie shared a picture of himself and Mr. Steck on Instagram, saying he was “totally gutted” and “beyond sadness” at the news.

Mr. Steck, who twice won the Piolet d’Or award, mountaineering’s highest accolade, attracted some controversy along with the acclaim. In 2014, after becoming the first person to complete a solo climb up Annapurna’s main south face, some people doubted his achievement because he could not offer independent proof that he reached the summit. He was also one of several European climbers who brawled with a group of Sherpas in 2013.

Mr. Steck had climbed Everest before, and he took its challenges seriously. In a report from his successful summit in 2012, he reflected on the dangers. But he also recalled his frustration at falling behind other climbers.

“Surely I couldn’t be more tired than the others,” he wrote. “From now on I had to fight. I convinced myself that reaching the summit was only a matter of determination. And I decided that I would reach the summit.”

Last Monday, Mr. Steck told his Facebook followers that preparations for what would be his last expedition were going well. And in his YouTube video posted before his climb, he described himself as “super ready, I mean I’m so psyched,” adding, “For me the project is already a success.”