Nonrunners often imagine that people can cover 26.2 miles only because they have lean, muscled legs and a highly developed cardiovascular system. Nothing could be further from the truth. The runner’s most important organ, by far, is the brain — the source of our dreams, drive and determination. Almost a century ago, the great Finnish distance runner Paavo Nurmi said: “Mind is everything; muscle, mere pieces of rubber. All that I am, I am because of my mind.”

Amby learned this while a teenager. The son of a Y.M.C.A. director, and skilled at football, basketball and baseball, he reached his limit in those sports in 10th grade when innate speed and power began to trump practice hours. He was, in a few words, too skinny to excel. Distance running offered an outlet and greater rewards for high commitment. Six years later, he won the Boston Marathon.

At different times and in different individuals, the mind of the marathoner ranges widely: from steely toughness, to sparks of creativity, to generosity on a grand scale. Sometimes, it surprises us.

In the first mile of the 1979 Boston Marathon, George fell into step with a Bowdoin College student, Joan Benoit, and they agreed to run together. As the race unfolded, Benoit held second place among the women, the spectators shouting, “Second woman, second woman!” George got caught up in the excitement. As the miles went by, he began to feel that he belonged there with his new friend even though his 44-year-old legs were moving faster than they ever had. Benoit went on to win the marathon, and George achieved a personal record.

In 1984, Benoit won the Olympic marathon trials 17 days after arthroscopic knee surgery. She captured the gold medal in the first women’s Olympic marathon several months later. To induce a relaxed, confident mental state while running on the steamy Los Angeles freeway, Benoit imagined herself home in Maine on one of her favorite coastal byways.

Running teaches all of us that goal-setting, persistence and tackling one mile at a time can lead to unimaginable achievements. Lessons are learned on the road, day by day, from personal feedback and experience.

As Dr. Jeff Brown, a Harvard psychologist and an author of “The Winner’s Brain,” said: “Negotiating a marathon requires many of the same mental characteristics needed in life. You have to control your emotions at times, activate your motivation when you’re down, and develop resiliency in the face of difficult conditions.”