One of those runners had a lot more to overcome than many others.
Gavin Maitland was diagnosed with a rare lung disease four years ago at the age of 40 and had to undergo a double-lung transplant.
"I went downhill real fast," Maitland said. "They asked me the typical questions like...'Are you a smoker?'... 'Do you have a family history of lung disease?' .... Were you exposed to any environmental hazards?' and the answer was 'no'."
Maitland was always a runner, but had never run a half-marathon, so he made it his goal to push through and prove not only to himself but also his 13-year-old son, Zander, that anything is possible.
"He taught me a lot of things... to keep going and don't be afraid," Zander said.
On Saturday, Zander ran the half-marathon with his dad.
"Before his transplant he [was a runner] and now he's getting back into it and I feel great to be a part of this and have the ability to run with him," Zander said.
Gavin says the transplant not only saved his live, but transformed it as well.
"I like to demonstrate that with a transplant you can still be very fit and active and live a normal life," Maitland said.
According to the American Transplant Foundation, the United States is facing a critical shortage in the number of organs available for transplant.
This year, an estimated 7,000 Americans will die waiting to receive a transplant that could save their lives.
In Colorado alone, there are over 2,200 people on the organ transplant waiting list, 48 of which are waiting for lungs.
In the past month, seven Colorado residents have been added to the waiting list.
Maitland hopes that his unlikely run will draw attention to the tragically long transplant waiting list, encouraging others to become organ donors.
To learn more visit the www.americantransplantfoundation.org.
(KUSA-TV © 2012 Multimedia Holdings Corporation)