For runners, one of the most storied-and feared-stretches is Heartbreak Hill, 32km 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 2:42:35, good for 241st overall. But in August, he became the first to complete what may arguably be the world's toughest 42.195km race: up the face of Kilimanjaro, the world's tallest freestanding mountain and the highest in Africa. In this debut run, which was measured to be an exact marathon distance, Rasmussen crossed five ecological zones, from bushlands through a rain forest and up to the glacier-capped summit at 5895 metres. It took him eight hours and 33 minutes.

Rasmussen's race up Kilimanjaro was a final test to experience a course he's measured in pieces in 2016 and run parts of before. This August, through his tour group, Erik's Adventures, he'll take a group of similarly adventurous runners up the same route. So far, 19 people, some of whom will hike the mountain a week earlier to avoid altitude sickness, have signed up.

More than 64 million people in the US ran or jogged in 2016, according to recent figures. As more runners than ever look to complete the Big Six - Tokyo, Boston, London, Berlin, Chicago and New York - one challenge is simply getting in. For next week's race in Boston, more than 5000 runners who qualified can'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.

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"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, organised events that more and more runners are going off road for a more adventurous 42.195 km. 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 42.195 km 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, there is more than one marathon in Antarctica.) 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 all of them. "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 1200 runners have completed all six through Abbott.

The more exotic the location, the higher the cost. The entry fee for Boston was most recently US$185 ($251) for US residents and US$250 ($340) 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 US$7000 ($9518) without counting the flight to Chile or Argentina.

Every extreme marathon poses unique challenges, whether surviving the gut-churning Drake Passage to run in freezing conditions in Antarctica or the steep ascents, descents, and countless steps to reach Machu Picchu in Peru. And every course incline is examined and discussed beforehand to prepare for the challenge.

Jeff Adams, president of Marathon Tour & Travel, started his career the way many of his runners did: on Wall Street. He gave up a 27-year career at Morgan Stanley to join the operator with which he previously travelled. In 1995, his company created the Seven Continents Club, the adventurer's bucket list to the road race circuit, with the addition of the Antarctica Marathon.

The organisation has been adding races, including one to Madagascar that "sold out within a week." Adams gets regular feedback from clients about what they want to experience next. In December, runners voted to add Bhutan, Cape Town, Jerusalem, and Patagonia to the offerings, Adams said. Next month, he's taking a group for the first time to Bhutan's Thunder Dragon Marathon as a way to see a place that limits tourism. Beforehand, a few will also go with him to the Great Wall Marathon in an extended "Dragon Challenge" trip.

New Yorker 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."

Indeed, one appeal of these ridiculous runs is not feeling as much pressure not to walk during them. Most of the runners who join up aren't elite; they're just going off-road to pursue their own adventures, Rasmussen says. "Being hard is part of the draw. Whatever you do is an accomplishment."

Upcoming 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 four kilometres to peak at 2529 metres up. The final nine kilometres winds through villages on a dirt road before opening up to a view of the iconic Taktsang Monastery, known as Tigers Nest and built into a cliff. The race fee is US$225 ($305); US-based Marathon Tours & Travel offers packages starting at US$3,866 ($5257) per person for a double -occupancy accommodation, but the cheapest options for the eight- and 10-day trips this year are sold-out.

Uganda International Marathon, June 2: This marathon made its debut in 2015 and has become a major fundraising force that aims to tackle poverty in the Masaka region. About 3000 will run this year. The event is set up as a seven-day adventure centred around volunteering and learning about the community, including helping organise an event for disadvantaged children. Organisers are accepting reservations for 2019, with options for 10-kilometre and half-marathon distances as well. All-inclusive packages start at US$945 ($1285), and the all-inclusive Gorilla Trekking packages start from US$1145 ($1557).

Leadville Trail Marathon, June 16: The Leadville Trail 160-km run in the Colorado Rockies 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 4018 metres, but you must expect to walk because the rocks up there are tricky. "If you are going to have a gruelling, tough, leg-busting, lung-busting race, it's this one," says Chlouber, 79. Race registration costs US$115 ($156), a half-marathon the same day is US$100 ($136).

Tusk's Safaricom Marathon, June 30: This running safari in Kenya has drawn elite runners, including Eliud Kipchoge, to the Lewa Wildlife Conservancy, about a 45-minute flight or four-hour drive north of Nairobi. Corporate teams are welcome and have included BlackRock Inc. and Investec. The entry fee for full or half is $285 pounds, plus a fundraising requirement of $2,100. Exceptional Travel offers a three-day package that includes a domestic flight from Nairobi, airport taxes, some meals, and hotel and safari tent accommodations for US$2550 ($3467) for two people.

Petra Desert Marathon in Jordan, September 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. The race limit is seven hours. Albatros offers a five-day package for runners that includes an entry to Petra and a dip in the Dead Sea for US$1150 ($1563).

Kilimanjaro Trail Marathon, Tanzania, August 13: The course starts with a descent before climbing 25.7 km 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. August is one of the best times to go; even then, the weather can swing wildly from T-shirt weather in blazing sun to hail and snow. Temperatures drop with the sun. Altitude sickness is a main concern, so acclimating ahead of time is highly recommended. There is no time limit, and rest stations will be available every three to four miles. Five-day packages from Erik's Adventures start at US$2795 ($3800).

Polar Circle Marathon in Greenland, October 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. Runners can also participate in the Polar Bear Challenge, which follows the Saturday marathon with a half-run the next day. Five-day packages are available directly through Albatros Adventure Marathons for US$2350 ($3195).