The British kayaker who was murdered 42 days into a daring trek on the Amazon River was constantly pushing her boundaries.

A former primary-school principal in London, Emma Kelty ditched her job in 2014 to become a full-time thrill-seeker — hiking 2,600 miles alone along the Pacific Crest Trail through the US and becoming the sixth woman to ski solo to the South Pole.

Kelty’s extreme adventures were part of a bucket list she’d made on the heels of her father’s death.

“Life is too short. But at least my father is reunited with my mother. But I miss them both — greatly,” she wrote on her blog in January 2015.

“OK, time to start on the bucket list!” she wrote in a more uplifting post three months later.

Kayaking 4,000 miles on the Amazon was just the latest box for Kelty to check off, but it would be her final adventure.

The 43-year-old Kelty was shot twice with a sawed-off shotgun and robbed by a band of pirates in a remote area near Coari, Brazil — a tragic ending she eerily foretold in one of her last Facebook updates.

“So in or near Coari (100 km away) I will have my boat stolen and I will be killed too. Nice,” Kelty wrote Sept. 10 along with three emoji sad faces.

She was killed three days later.

Born in London, Kelty led a double life — one of strict routine as the head of primary schools in Hackney and Surbiton and another as a nature-obsessed adrenaline junkie, skydiving more than 90 times.

“My outside adventures were the yang to my work, but in so many ways it mirrored it too,” she wrote on her Web site. “I’ve spent many a holiday in Scotland and like nothing better than to walk the mountains. I always saw this as my sanctuary away from a very demanding and focused career that spanned across the education system.”

In September 2014, she quit her job as a professional educator for good, telling colleagues she was leaving for an unknown “period of time,” according to the Telegraph.

“She effectively decided she was going to dedicate herself to the world of adventuring,” her friend Masha Gordon told The Post.

Neighbors said Kelty sold her north London flat about two years ago to fund her daring life life. Turning to travel was a way to get through a rough patch that included the loss of her father to cancer.

Just before embarking on her US hiking trip, she wrote about hoping it would “give me space to mourn my father who was a key person in my life but up until this point, due to other factors/happenings in life, I haven’t been able to so.”

The fiercely independent adventurer also opened up about using her “solo time” to do some soul-searching.

“There is so much that I want to say about this remote solo time — it has been a comfort but equally it has pushed so many boundaries, fears but also reaffirmed many things too,” she penned while journeying to the South Pole.

It was on her latest excursion, to the Amazon, that Kelty expressed finally being at peace with herself.

“I couldn’t be happier,” she wrote in her final blog post on Aug. 9. “Long gone are the days of working day and night, weekends and holidays and I am defiantly [sic] a better person (personally) for it — I recognize that for many years I gave myself to others too much that somehow I lost myself in it.”

Kelty threw herself into a life of danger. A year after quitting her job, she survived a four-month, 2,600-mile hike of the Pacific Crest Trail across North America, starting at the US border with Mexico and ending in Canada.

Soon after, she set her sights on the South Pole and traveled to New Zealand to learn how to cross-country ski. In early 2016, Kelty completed a 600-mile guided ski trip from Union Glacier to the South Pole — and a year later became the sixth woman to ski to the South Pole solo.

During her historic 46-day trip, Kelty suffered a lung infection and a debilitating injury known as “polar thigh” amid temperatures of minus-40 degrees Fahrenheit.

“She must’ve been in horrendous pain, and she soldiered on,” said Gordon. “Then she found a new challenge.”

Like skiing, Kelty was brand new to white-water kayaking before she began planning her Amazon trip in February. The brave Briton prepared by taking paddling lessons, white-water safety courses and practicing on the Thames River and in the waters of Wales.

She also took a self-defense class and regularly hiked to get herself in shape for the grueling summer journey.

“Emma was a woman with a dream for adventure and the boldness to embark on journeys that few in our world would ever muster the courage for,” said Rocky Contos, who accompanied Kelty on the first leg of her kayaking trip that began in mid-June.

Kelty was determined to make it across the 4,000 miles of the Amazon alone — beginning in the Marañón River in Lima, Peru and ending at the Atlantic Ocean — and set no specific deadline to get it done.

“I want to do this as a challenge for the next step to what I hope to be my next adventure that will take another leap in the ‘challenging the boundaries’ adventure,” she wrote.

She set out to kayak 15 miles a day — but learned six weeks into her trip that she was surpassing that goal by clocking more than 31 miles daily.

The arduous paddling left her exhausted at the end of the day, when she’d camp along the river banks and be greeted by locals.

“Too many broken nights sleeps . . . getting more tired every day . . . . lets hope I dont get 4 more guys ‘returning to visit me after dark’ tonight . . . 54km done,” she wrote on Aug. 23.

Kelty’s frequent posts on social media about her travels pointed to brushes with danger — and her general nonchalance over them.

In May, she wrote about receiving warnings that she was risking her life by setting out on the Amazon.

“I have had folks, friends and strangers telling me not to do it, its stupid, its too dangerous . . . its too risky and I will die,” she blogged. “Significant time has been put in to processing and boxing those comments and sorting the comments into the . . . ‘I told you so’ group.”

A day before she disappeared, Kelty told her Facebook followers she’d encountered gun-armed pirates.

“Turned corner and found 50 guys in motor boats with arrows!!!” she posted. “My face must have been a picture!! (Town was uber quiet . . . too quiet!!)”

She then added, “OK 30 guys . . . but either way . . . that’s a lot of folks in one area in boats with arrow and rifles.”

Her final post came on Sept. 13.

“Such a dramatic change in one day . . . but such is the river . . . . every km is different and just because 1 area is bad doesn’t mean next is,” Kelty wrote with a smiley emoji.

Kelty was camping on the island of Boieiro, a remote area between the towns of Coari and Codajás in Brazil known for drug trafficking, when she was ambushed by seven pirates.

She was shot twice and still alive when the bandits hauled her onto their canoe and cut her body into pieces, one of the suspects told police, according to Ivo Martins, the head of the Amazonas state police homicide division.

Kelty’s remains were tossed in the Solimões River and haven’t been found.

The robbers made off with her two cellphones, GoPro camera, tablet computer and drone, which they tried selling in neighboring towns, police said.

Her kayak — which the suspects tried stabbing holes into, according to local media — was found near her last campsite. “She had no chance to react to what happened to her,” Martins said.

“The gang operates like pirates and go out in a canoe looking and hunting for the chance to steal. It appears they did not have a predefined victim.”

Three of the suspects, including two 17-year-olds, were nabbed Monday. Police are hunting the rest.

In February, Kelty told the BBC that her white-knuckle excursions were worth risking her life — but believed she’d be ready in the face of danger.

“Oh, yes, I mean that’s half the challenge,” she said.

“But it’s about minimizing the risk. I’m going to a self-defense course, which is going to be tailored to de-arming people, so if I do come across that situation at least I’m prepared for it.”

Additional reporting by Isabel Vincent