Before Guli set off on her 100-marathon quest, doctors told her it was too dangerous.
Guli had done back-to-back races as a way to raise awareness about water issues before: In March 2016, she ran 40 marathons in seven weeks across seven continents. About a year later, she ran 40 marathons in 40 days.
She has said that running around the world helps her better experience water-starved communities first-hand.
Despite this experience, Guli's doctors warned that her age and agility would pose a challenge. She trained for the 100-marathon quest anyway, relying on a mix of daily runs, swimming, and cross-training.
"I've been advised against a lot of the things I’ve done in my life," Guli said. "If we listened to all the excuses, they would become excuses for us, too."
Guli experienced what it's like to go without water during a 10-year drought in Australia while she was growing up.
She became accustomed to water-conservation measures like taking short showers or shutting off the tap while brushing her teeth.
"Dinnertime conversations weren't about the economy," she said. "They were about how we were saving water."
The experience formed the foundation for her career as CEO of Thirst, a global initiative from the World Economic Forum that educates young consumers about water scarcity.
By 2030, the world could see a 40% greater demand for available water than its existing supply.
As CEO of Thirst, Guli learned how much water is used in our supply chains.
"Look at the levels of water falling in Lake Mead," she said. "That doesn't just affect the people living in California. It affects every single person who goes to a café in New York and buys almond milk, because the vast majority of almonds are grown in California."
Guli said something she saw during her 2016 set of marathons stuck with her and inspired her to attempt an even more ambitious physical feat.
During that trip, Guli stopped at the Richtersveld desert in South Africa, a nation whose demand for water could exceed its supply by 2025.
While crossing the Orange River, which borders South Africa and Namibia, Guli said she saw people wading through knee-high water. The water levels used to reach an area about 23 feet above the riverbank, she learned, but the river had been drained to grow grapes.
Seeing that contrast, Guli said, reinforced her drive to call attention to the water crisis.
Guli dubbed her 100-marathon campaign #RunningDry, and picked routes through areas that had been severely impacted by the water crisis.
At the beginning of her challenge, when Guli was still completing full marathons, she ran near the Aral Sea, a lake between Kazakhstan and Uzbekistan that was once the fourth largest in the world. Today it's almost completely dry.
In the 1960s, the Soviet Union drained the Aral Sea's feeder rivers to irrigate crops like cotton and rice. Over time, the lake began to dry up, destroying fishing communities.
During her marathon in the area, Guli ran past the carcasses of deserted ships that were left to rot on dry land.
Today, the little water that remains in the Aral Sea has been contaminated with fertilizers and pesticides from local farms. Frequent dust storms blow these toxic chemicals into the air, creating a health hazard for residents.
After running across Europe, Asia, and the Middle East, Guli returned to South Africa for what would turn out to be her final run.
One of her first visits after landing in Cape Town was to Beaufort West, a town with some of the most dire drought conditions in the world. There, government trucks distribute 2.6 gallons of water a day to local households, which typically have around three to seven residents.
Guli rode on one of the water-delivery trucks, and said children and teenagers chased after it, begging for water. Some residents even broke out in arguments over where the trucks were headed.
"If ever there was a salutary lesson in what our future looks like unless we act now, it was Beaufort West," Guli said.
While in South Africa, Guli began walking her marathons due to an injury. When she realized the extent of her diagnosis, she worried her trip had come to an end.
"I thought that the whole campaign was over," she said. "I thought the thing was finished and everything I worked for was falling down."
But members of her support team offered to run on her behalf, and people around the world began to "donate" miles to the campaign. Guli opted to continue touring as opposed to running marathons.
She visited California's Salton Sea in January, near the end of her campaign.
California's biggest lake, the Salton Sea, once got more annual visitors than Yosemite. Now, the surrounding area is strewn with the remains of old homes and boats. Some residents even claim there are fish bones in the sand.
Guli said locals are worried the Salton Sea could become the next Aral Sea.
"It looks the same. It feels the same," she said. "The people's concerns are the same."
The lake continues to shrink as California diverts water from the Colorado River toward coastal cities. As the soil dries, the lake-bed is expected to release toxic dust that could lead to more asthma cases in the region.
On day 100, Guli crossed the finish line at Central Park in New York City.
A group of supporters ran a full marathon in New York, while Guli participated in a wheelchair. But she crossed the finish line on foot.
"I [didn't] know how I would feel," she said. "I always envisaged that I would be the one to run the last [26 miles]."
By the end of her campaign, Guli said, her supporters had collectively run more than 810 marathons.
Guli said a 5-year-old ran a few miles and even a few 80- and 90-year-olds clocked their times.
"These are things that I never would have envisaged sitting in that wheelchair," she said. "The people on my team were right. This is much bigger than me."
She's now gearing up for her next event: a campaign for World Water Day on March 22.
Guli said people don't have to run any marathons to support her water-awareness campaign.
"It's extremely easy to feel disillusioned and disenfranchised," she said, referring to the water crisis.
But Guli said even remembering to shut off the faucet could make a huge difference. In September, Colgate (Guli's sponsor for the #RunningDry campaign) estimated that if everyone exposed to its water-saving message turned off the tap while brushing their teeth, the world would save more than 50 billion gallons of water per year.
"Someone once said to me, 'If climate change is the shark, water is the teeth," Guli said. "It's the place where we’re going to get bitten first.'"
