(Reuters) - Last time she ran 40 marathons in a row, Mina Guli wore out eight pairs of shoes. Now she is trying it a second time.

The 46-year-old Australian set off on Wednesday morning in Las Vegas on the first leg of 40-day, multi-continent journey. She hopes to inspire people to safeguard the world’s freshwater supplies over the course of a 1,048-mile (1,687-km) run, loosely following the banks of six major rivers.

The start of Guli’s effort coincided with Wednesday’s World Water Day on the U.N. calendar. Guli runs Thirst, an international charity that seeks to raise awareness of water-scarcity issues.

By early afternoon, Guli said she had finished just under 20 miles, out of 31 she planned for the day, when she stopped to take a phone call from a reporter.

“It’s nice to be able to actually get out here and do some running,” Guli said, adding that she had been busy dealing with logistics in the days leading up to her departure. “I’m slightly intimidated by the task that I’ve taken on. It’s quite daunting.”

Last year, Guli finished an even longer run - connecting 1,055 miles of terrain across seven continents. Guli said both runs are to raise awareness of the amount of water that goes into consumer goods.

“For example, most people don’t know that the water that went into the clothes they’re wearing ... took more water to make than all the water they’ve drunk in their entire lifetime,” Guli said.

After running along the Colorado, Guli will go on to the Amazon, then Murray River in Australia, China’s Yangtze River, then the Nile, before finally finishing on the Thames April 30.

Honestly the whole thing is a huge challenge,” Guli said. “But every time that I think that, I think about the reality of the water crisis, and how big that challenge is. What I’m doing is small in comparison.”