What drives a person to take on the personal challenge of a marathon? It's 26.2 miles of pavement pounding under what can sometimes be brutal weather conditions.
For many, it's a personal challenge to themselves, but many others are driven by selflessness.
For the last two weeks, NBC Chicago has profiled several runners and organizations that will be among the thousands traversing through 26 Chicago neighborhoods on Sunday.
We also put out calls on our Facebook and Twitter accounts to learn of other heroes and inspiring stories.
Eileen Quinn, 28, Chicago
"I'm running my first marathon in honor of my Dad. He was a 5-time Chicago Marathon runner and he passed away a year ago. In my long training runs when I want to give up, he is the first thing my mind goes to. I know he'd be thrilled that I am doing this and I wish he was around to cheer me on."
David J. Haffner
Haffner has already ran three marathons and is making Sunday his fourth. He said he was struck by a "mixture of excitement and memory and fear" when a fellow nurse asked him if it was hard to run the race.
"Excitement won that day, and I signed up," he said.
This year, Haffner said he qualified for a faster start corral, which was a confidence booster.
"But it's a trap, I tell you. Then you're like, 'Could I get into the next corral up next year? So I'm in and I might be ready. I'm never ready til' I'm on the starting line. Cause' then you're like a bullet, nothing left to do, nowhere left to go but ahead. Through this beautiful city of Chicago with a bunch of your family."
Wro told NBC Chicago her hero was her friend Joe Massow, who has run the Bank of America Chicago Marathon for the last four years and has raised more than $40,000 to fight leukemia and breast cancer as well as supply toys to Children's Memorial Hospital and Comer Children's Hospital at the University of Chicago.
Massow is motivated by the loss of his best friend, who died in 2010 to leukemia.
Ratell will be running Sunday in his first-ever marathon. Driving him: his 5-year-old son, Brendan. Brendan is living with a genetic disorder called Neurofibromatosis, which causes tumors -- both benign and malignant -- to grow.
His parents have made it their mission to raise awareness and money to fight the condition. Ratell took on the personal commitment to run in honor of his son and trained alone, all while working full time and taking care of his family.
Ratell has been raising money but is a bit short of his $5,000 goal.
Brennan says her father, Fernando Pedroza, has become addicted to the adrenaline he feels while running. Sunday's race will be Pedroza's 15th marathon, and he says he's motivated by the beer he knows he'll get soon after crossing the finish line.
|Team World Vision is an organization aimed with providing clean water for kids living in poverty in Africa.|
|Team Paws Chicago is running the 2011 Bank of America Chicago Marathon to support the area's largest no kill adoption and humane organization.|
|Team Sweetness: This year the Walter Payton Foundation and Jarrett Payton Foundation team up to help enrich the lives of youth through charity fund raising.|
|Two-time women's wheelchair champion Amanda McGrory shares her educational and inspirational story.|
|Allen Chong says he returned from a tour in Afghanistan with the Illinois National Guard lacking goals. He set several, and is running the 2011 Bank of America Chicago Marathon to benefit Catholic Charities.|
|Mingo Perez is running his first marathon with the group "Back on My Feet." The nonprofit organization promotes the self-sufficiency of those experiencing homelessness by engaging them in running as a means to build confidence, strength and self-esteem.|
|Girls On The Run's mission is to inspire young girls to be joyful, healthy and confident through running|
|Mark Johnston says he's running in the Bank of America Chicago Marathon for Ronald McDonald House Charities to give back to families who have children in the hospital.|
|Jenn Peterson says she's competing in the Bank of America Chicago Marathon for Friends of Prentice, which helps support uninsured women and premature babies.|
Coverage of Sunday's race begins at 7 a.m. on NBC 5 in Chicago. Live video will also be offered on NBCChicago.com, NBC Chicago Nonstop, NBC Chicago's Facebook page and on mobile devices.