The 48-year-old doesn't have a passion for racing, or even for running. In fact, Guli considers herself "a bad runner," she tells CNBC Make It. "I get injured all the time, and I'm old." She runs for one reason: to raise awareness of the world's water crisis.
Running marathons is "my way of representing just how big this water crisis is," says Guli, an Australian corporate-lawyer-turned-activist who founded Thirst in 2012 to educate the next generation about the importance of water conservation. Her organization collaborates with over 1,000 schools, has hundreds of volunteers teaching its education programs and has graduated more than 1 million students from its programs.
Beyond raising awareness through her 100-day running initiative, she hopes to meet with communities directly affected by the water shortage around the world to better understand both the crisis and how to solve it.
From New York, Guli will head to Europe, and then Uzbekistan, India and China. She'll run across parts of the Middle East, Africa, Australia and South America before finishing her 100-day challenge back where she started, in New York City, on February 11, 2019.
If Guli goes through with her plan, she'll run 2,620 miles in 100 days. It won't be the first time she's pulled off something physically extreme: In 2016, she ran 40 marathons across seven deserts on seven continents in seven weeks. In 2017, she upped the ante and completed 40 marathons in 40 days.
But "even that is not enough," she says.
"I talked to a lot of people about what it means to be committed to something and I realized that to achieve anything — especially to achieve big things — we need to be 100 percent committed to them, and so the idea came up about running 100 marathons." In early 2018, the ultra-runner and water activist teamed up with Colgate, who is sponsoring her "#RunningDry," initiative.
The NYC Marathon, which she finished alongside 52,700 other runners, is the only organized run she's completing. She'll do the other 99 on her own time.
"What normally happens is that I often start running alone," says Guli. But over the course of the the 26.2 miles, community members tend to join in and run alongside her, making her "feel a bit like Forrest Gump."
Ideally, Guli will start and finish her marathon in the morning so she can dedicate the afternoon to meeting with community leaders and organizations, visiting schools and engaging with the locals. But, thanks to her busy travel schedule, she'll never have a consistent routine.
"Sometimes, like in Australia, when we land at 3:30 in the afternoon, we'll have to start running after 3:30 p.m. and finish before midnight," says Guli. "There are a huge number of logistics."
While she admits to being "totally terrified about the whole thing," she remains focused on her goal. "The thing I'm most afraid of is not affecting the change I dream of — and that's solving our water crisis."
In 2015, the World Economic Forum ranked the water crises as the biggest threat facing the planet over the next decade. "In many countries, we are using water faster than nature can replenish it. We are reaching a breaking point," the Thirst website notes. "Without changes in behaviors and business practices, by 2030 demand for water will be 40 percent greater than supply."
The response to her activism hasn't all been positive, Guli says. "I've had a lot of people tell me, 'You're not an athlete. How can you possibly do this? Nobody's ever done it before. You can't do it. Who are you to think you can run 100 marathons and change the world?'"
But she generally shrugs off the criticism. "There are a lot of people out there who will tell you the negatives. I think that it's easy to listen to negative voices if you want to find excuses — but nobody who ever changed the world listened to those excuses. Nobody who ever did a marathon listened to those excuses. Nobody who ever achieved their dreams listened to those excuses.
And her response to skeptics is to tell them "a couple of things," she says. "The first is: Watch me. The second is: Yes, I'm crazy. But sometimes crazy changes the world."
Like this story? Subscribe to CNBC Make It on YouTube!