For runners, one of the more storied - and feared - stretches is Heartbreak Hill, 20 miles into the Boston Marathon. The 90-foot elevation gain over one-third of a mile would be a painful climb in any marathon, but here it's compounded because it occurs at the distance where many runners hit the proverbial wall.

Last April, Erik Rasmussen, a 42-year-old trail seeker, runner and triathlete, finished the Boston Marathon at 2 hours, 42 minutes, 35 seconds, good for 241st overall. But in August, he became the first to complete what may be the world's toughest 26.2-mile race: up the face of Kilimanjaro, the world's tallest freestanding mountain and the highest in Africa. In this debut run, measured to be an exact marathon distance, Rasmussen crossed five ecological zones, from bushlands through a rainforest and up to the glacier-capped summit at 19,341 feet. It took him 8 hours, 33 minutes.

More than 64 million in the U.S. ran or jogged in 2016, according to recent figures. As more runners look to complete the Big Six - Tokyo, Boston, London, Berlin, Chicago and New York - one challenge is simply getting in. For Monday's race in Boston, more than 5,000 who qualified couldn't run because of the space limitation. Roughly 100,000 applied for the New York Marathon in November, but only 51,000 lined up for the starting gun.

"In the last 10 to 15 years, running a marathon has gone from something extraordinary - that the very few did - to a popular trend that the average person would take on as a challenge," says Steen Albrechtsen, spokesman for Albatros Travels in Copenhagen.

City marathons have become such massive, organized events that more runners are going off road for an adventurous 26.2 miles. In recent years, travel companies have expanded running packages to the Arctic Circle, the Great Wall of China, the Petra archaeological complex in Jordan, and the Bagan Temples in Myanmar. These are not "ultra marathons" in which runners compete for extended distances. They are rigorously measured 26.2-mile courses - minus the massive crowds, well-stocked snack tables and well-paved asphalt. Rasmussen calls them "adventure marathons."

They, too, are selling out in record numbers. Capped at 50 to 100 runners in areas for environmental and safety reasons, there's already a two-year waiting list to run the Antarctic Ice Marathon, the world's southernmost race. Marathon Tours & Travel is confirming runners for its Antarctica Marathon for 2021. (Yes, Antarctica has more than one marathon.) The 50 spots in Peru's Inca Trail Marathon in July have been reserved for months. Albatros's Polar Circle Marathon in Greenland is almost fully subscribed ahead of its October date.

"Runners in general are goal-oriented people," says Tim Hadzima of the Abbott World Marathon Majors, which runs a series promoting the "Big Six" marathons and awards a medal to those who finish them all. "Once folks run a race, they usually want to do another one and do another one and another one and another one." So far, more than 1,200 runners have completed all six through Abbott.

The more exotic the location, the higher the cost. The entry fee for Boston was most recently $185 for U.S. residents and $250 for international participants. Costs can add up in any location, depending on travel and hotel needs. A trip to Antarctica, by boat or plane, can easily reach $7,000 without counting the flight to Chile or Argentina.

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Sharon Venturi, 42, ran in Antarctica through Marathon Tours & Travel last month. She found the other runners to be less competitive and more supportive than those in big-city races. "It's a totally different approach," she says. "Everyone is dedicated to a healthy lifestyle and adventurous - not about times and competition. Though there is still a little of that anytime a time is being kept." Extreme marathons

- Bhutan's Thunder Dragon Marathon, May 27: The marathon starts on a downhill, passing over a metal swing bridge and along the Paro Chu River in the Paro Valley before climbing gradually above rice paddies. Then it gets hard. After the halfway mark comes a steady climb for 2.5 miles to peak at 8,300 feet up. The final 6 miles winds through villages on a dirt road before opening to a view of the iconic Taktsang Monastery, known as Tigers Nest and built into a cliff.

- Uganda International Marathon, June 2: This marathon made its debut in 2015 and has become a fundraising force that aims to tackle poverty in the Masaka region. About 3,000 will run this year. The event is set up as a seven-day adventure centered on volunteering and learning about the community, including helping organize an event for disadvantaged children.

- Leadville Trail Marathon, June 16: The Leadville Trail 100-mile run is an icon on the ultra-marathon circuit, a run so epic that founder Ken Chlouber, a 14-time finisher, started a separate marathon for those who wanted the challenge without the distance. The latter race is an out-and-back "run" up to Mosquito Pass at 13,185 feet, but you must expect to walk because the rocks there are tricky. "If you are going to have a grueling, tough, leg-busting, lung-busting race, it's this one," says Chlouber, 79.

- Petra Desert Marathon in Jordan, Sept. 1: Starting at the ancient city of Petra, this race is a climb that takes you over paved roads, sand and gravel, across river bends and stretches beyond rock formations that look as if they belong on Mars. The good news is that the race ends on a downhill, albeit a steep one.

- Kilimanjaro Trail Marathon, Tanzania, Aug. 13: The course starts with a descent before climbing 16 miles to the highest point in Africa, then sharply plunges from the summit on a course that feels as if you're sledding through volcanic gravel on your feet.

- Polar Circle Marathon in Greenland, Oct. 27-28: Most of this Arctic marathon occurs on gravel that's frequently snow-covered. A portion will take place across the ice cap in the polar circle. It's got hills and weaves around the ice sheet. Temperatures will be at or below freezing, with strong winds, and the race is capped at 250 contestants for logistical and safety reasons.