The journey starts at the edge of Boulder Reservoir.
"This will be our first crack at 140.6," David Slomkowski said.
That's 140.6 miles he's referring to. 2.4 swimming in the Reservoir, followed by 112 miles on the bike.
"It's hard," Davis said. "The bike is probably the hardest of the 3 disciplines we do."
Then there is the marathon. 26.2 miles on foot.
"Never thought in a million years I'd be doing one of these things much less James, much less doing it together," David told 9NEWS.
He and 18-year-old James Banks are in this as a team. James lives with Cerebral Palsy and Scoliosis and would never be able to complete a race like this on his own.
Like so many others, their Ironman goal has nothing to do with the clock.
It's about figuring out what is possible.
"Who cares about a time, but to cross the finish line would be a dream come true," David said.
Slomkowski is the founder and executive director of Athletes Serving Athletes. The organization pairs people with disabilities with able bodied athletes to tackle all sorts of events. James will ride in a raft tethered to David's waste for the swim. They have a tandem bike for the ride and a specialized racing chair for the run.
"Never give up. Never quit," James said. That's the motto. The two have been at this for 10 years now and James says David is like a father to him.
"He's taken me to places that I'd never in a million years have the opportunity to," said David fighting back tears. "I certainly wouldn't be at the front of an Ironman triathlon in Boulder this weekend."
An emotional couple of days for him.
"It's just been a combination of 10 years of hard work from everyone in our organization," David said wiping tears from his cheeks.
More tears are likely if the duo gets to the finish line on Sunday night, as they try to change the answer to that question:
What is possible?