(CNN) For some, running just one marathon is the ultimate sporting achievement.
For others, it's just the beginning.
Amongst running communities, there are rare breeds who aren't so much concerned with the distance -- it's more about where they run 26.2 miles, who they meet and the memories they make on the way round.
That's Henrik Brandt's philosophy, and that's why he's run the Great Wall Marathon in China every year for the past 18 years.
When the race was founded in 1999 with only 350 entrants, Brandt was there. A combination of stunning scenery, testing conditions, and a great atmosphere means the Dane hasn't missed a race since.
"It's my favorite, my number one," Brandt told CNN.
"I've run marathons all over the world on every continent, I've had a lot of adventures over the years, but there's no doubt my love is the Great Wall Marathon."
As endurance races go, the Great Wall Marathon is unique. Covering 5,164 steps, you can find yourself ascending hundreds of meters in a matter of minutes, toiling in temperatures of up to 40˚C.
But there's something about the endless incline, the intense heat, and the camaraderie on the way round that runners find alluring, and Brandt is no exception.
It was the Great Wall Marathon that made Brandt, who only started running two months before he first competed in the race, fall in love with the sport.
"In 1999 I was married to my first wife," he explains. "She had run two marathons at that time and I thought she was just crazy. All the training she had to do before -- I thought it was a waste of time.
"We read in a Danish newspaper about the Great Wall Marathon, and I thought it was a great idea because it would be fun to visit China and the Great Wall and all the things I've heard about it."
A phone call later, and Brandt, to the disbelief of his wife, had signed himself up on a whim.
"She looked at me and thought I was mad. But later that day she went for a run and I went with her, and continued that for some weeks. I was not a runner at all.
"It was quite a challenge to start with the Great Wall Marathon. Then I finished in a better time than her, so I thought I'd have to try a real marathon to see what the difference is in the time.
"I signed up for the Berlin marathon four months later and, to give you a sense of how hard the Great Wall is, I did it two hours faster."
This year's winner won in a time of three hours, 14 minutes, 32 seconds, over an hour slower than the time it took Kenya's Daniel Wanjiru to win this year's London marathon.
The Great Wall Marathon has seen close to 23,000 finishers over its 18 years, a success that has lead Albatros -- a Denmark-based travel company that specializes in adventure holidays -- to broaden its marathon portfolio.
Albatros organize five different marathons in total and aim to offer something different to most organized marathons, swapping cities for safaris, concrete streets for stunning scenery, and well-trodden routes for remote resorts.
They appeal to athletes with a sense of adventure that the regular marathon circuit can't satisfy.
"Going to the Berlin marathon, which I've been to twice, I drive to Berlin by myself, book the hotel by myself, and get to the start area," says Brandt.
"I run in the morning then go back home [in Denmark] again -- I'm not really getting contact with all the runners at the Berlin marathon."
Raquel Holgado, a Sydney-based personal trainer who made a video of her Great Wall Marathon this year, agrees.
"I prefer new marathons, rather than repeating the same one," she told CNN. "But with the Great Wall Marathon I'd 100% do it again."
"It's great and completely unpredictable. You completely forget that you're doing something difficult or challenging.
"This was great because I did the video and had lots of people sending me emails and messages. It's just a great way to meet people."
On top of the Great Wall Marathon, Albatros also organizes one in the Arctic Circle; one in the ancient deserts of Petra, Jordan; The Big Five Marathon, held in a South African game reserve, and the Bagan Temple marathon that winds through the iconic spires of the Buddhist temples in Myanmar.
Whichever they sign up for, runners will be pushed to their limits -- be it by icy winds, energy-sapping sand, or sweltering desert heat.
"With the Big Five Marathon on the sand, you feel like you take three steps forward and one step back," Steen Albrechtsen, head of production at Albatros, told CNN.
"In the Polar Circle Marathon, we start on the ice cape and we run on the ice cape a certain distance and finish on normal roads. It's extremely cold.
"But I think the Great Wall might be the hardest. We always say that you can't win on the wall, but you can lose on the wall. You can't really push yourself. If you start pushing yourself, the unrelenting incline will get to you later on.
"It's a mix of temperature, terrain, and incline that makes them all so hard."
That said, it is meant to be a holiday, and Albatros wants the experience to be rewarding, if not necessarily enjoyable.
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"We try to make a race that's a 30% increase of someone's normal marathon time," Albrechtsen adds.
"We're not trying to break anyone. We're trying to give them a good race, we're trying to give them a good experience. We actually have something like a 97% completion rate."