There's no question that running a marathon is a test of human endurance.

Some run for fun. Others take part because of a personal challenge. Others run for charity.

Whatever their reason, thousands of runners have a spark of inspiration that makes them decide to take on the task of pounding the pavement for hundreds of miles in training and an ultimate 26.2 miles on race day.

And that, in turn, inspires others.

We asked NBC Chicago readers for some of their inspiring stories. We heard from runners and those who call them heroes.

Brenda Knox called her son, Alex Knox, a "very purpose-driven young man." Sunday's race will be the 16-year-old Fremd High School student's first marathon, and he's running to raise money and awareness of Team Salute.

Brenda said her son hopes to attend a service academy after graduation and become further involved with Salute, Inc., an organization which helps military service members, veterans and their families.

"As a family, we’ve always been patriotic but Alex’s interest has encouraged us to become more involved and aware of this worthy cause," she said.

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Nicci Martin is "the most heroic person," her husband, Steven Martin, told us. The University of Illinois Chicago police officer's training was sidelined when she was dragged and thrown from a car while helping a 19-year-old woman who was being attacked.

"She is currently going through physical therapy for her injuries to her left knee, shoulder, neck and back, as well as having suffered a concussion. She hopes to be recovered enough to run the marathon," Steven wrote. "She had been training with Chicago Area Runners Association, and raised more than one thousand dollars towards Girls on the Run."

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Kelsey Hanrahan wanted to make sure we knew of her friend, Gail Alwan, who continues to run marathons and do triathlons as a breast cancer survivor.

"It is not just about time to her or for the glory of sport, but rather to show herself and others that she is strong," Hanrahan said.

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Annette Martin professed adoration of her father, 68-year-old Amado Martin, who will be running his first marathon on Sunday as a part of his quest to cross another thing off his bucket list.

Annette said her parents came to the United States in 1973 and worked hard to assimilate to American culture and send four children through college. One the kids had an education, the parents wasted no time enjoying life: learning to ski, taking up ballroom dance and running half-marathons.

"He is truly my hero because he really lives life to the fullest and doesn't believe age should prevent you from doing anything your heart wants," she said. "To be honest, I'm also quite jealous that he has healthier knees than me."

By the way, Martin said her 70-year-old mother continues to run half-marathons.

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Beth Rathjen said she's been inspired by two of her family members: her aunt, Katie Peterson, and Peterson's daughter, Georgia Peterson.

Rathjen said her aunt's selfless nature has turned into her life's work. Sunday's race won't be the first for the Peterson, the Maryville School social worker who aims to help people to use therapeutic services such as yoga, meditation and art rather than medication.

"She has helped me in a way that I appreciate more than she realizes," Rathjen said.

Georgia Peterson is a sophomore at Elmhurst College and is running to raise money for Best Buddies. The organization's website says it's "dedicated to establishing a global volunteer movement that creates opportunities for one-to-one friendships, integrated employment and leadership development for people with intellectual and developmental disabilities."

"Georgia has always been such a hard worker throughout high school and now college," said Rathjen. "She was in the student council, played basketball and softball and excelled at her work too. Now she wants to help people with disabilities as her career. That is a hero for sure! She may be my younger cousin, but I totally look up to her as a person!"

Christy Smith says it's her friend, Jill O'Brien Dine, who's an inspiration. The mother is determined to find a cure for diabetes since her son, Clayton, was diagnosed. Sunday's race will be O'Brien Dine's second marathon within a month.

While O'Brien Dine may inspire Smith, it's the young boy who's the core.

"I also never thought that I would be so determined to find a cure for a chronic disease or that a chronic disease would be such a huge part of my life. Lastly, I never thought I would be so in awe of a five year old boy," O'Brien Dine wrote on Facebook. "Clayton is the strongest, bravest, funniest, most loving person I have ever met. If I had to be prodded and poked all day long every day, I would be pretty bitter. He does it with a huge beautiful smile! I am almost to my goal!! We will find a CURE!!!"

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Finally, there's Kari Thicksten. She's now a runner herself, but she said it's her husband, Mike Thicksten, who inspires her. Mike used to be a runner but lost the ability -- and a scholarship -- to injury 15 years ago.

During last year's marathon, Kari Thicksten said her husband was there with the children, cheering from the sidelines. It was there he made the decision to get back into the sport.

Mike's training for Sunday's race has been a secret from family.

"I think he was so afraid of committing and not being able to actually do what he wanted to," Kari said. "There have been ups and downs, many days were he didn't think he would make it to the starting line. He feel on ice during a January run, injured his knee and IT band, but through physical therapy and perseverance he has pulled through."

While Mike has a life-long love of running, he's not just running the race for personal accomplishment. He's also running for Team World Vision to help provide clean water for communities in Africa.

Thousands of runners will approach the starting line on Sunday, each with a personal story of how they got there. But marathons just aren't for those taking part. The runners' inspiration knows no bounds.

Feeling inspired? There's another Chicago Marathon next year.