She was young, spirited and rich. It was the 1970s and Kris McDivitt seemed to come straight from California central casting; the glamorous ski-racing daughter of an oil-industry man who made her fortune as the first CEO of what was to become the billion-dollar outdoor clothing company Patagonia.

And then in 1993, aged 43, Kris McDivitt unexpectedly fell in love with Doug Tompkins, the adventure-junkie rock-climber and deep green environmentalist who had co-founded not one but two giant outdoor-clothing companies, North Face and Esprit.

They married (she changed her surname) and the eco-power couple cashed in their shares and uprooted from California to live the wilderness dream in the real Patagonia where they went off-grid, flying between homes in the forests and mountains of Chile and Argentina.

Doug Tompkins had made $150m from the sale of Esprit alone, and Kris had made millions running Patagonia for 20 years. The idea was for each of them to set up a large conservation charity, combine their fortunes and buy up – and then restore – millions of acres of threatened wilderness.

Patagonia had it all: ancient forest, wetland, soaring mountain, fjords, coast, glaciers and active volcanoes. Their plan, says McDivitt Tompkins, talking during a trip to London, was to buy and restore as much land as they could, improve and protect it, and then return it to people as public, national parks.

So they bought 2.2m acres. But then, nearly a year ago, the remarkable love story of the world’s two wealthiest private conservation philanthropists abruptly stalled. After nearly 24 years’ of working together to own and protect more land than any other individuals in history and having together spent an estimated $375m, Doug died in a kayaking accident in Chile, leaving Kris with mountain, forest and steppe, but utterly bereft.

“His last trip? Compared with everything else he had done this was a cake-walk,” she says. Doug had been on a five-day kayaking trip on General Carrera Lake in the heart of the Andes mountains with Patagonia founder and owner Yvon Chouinard and other experienced kayakers. On the third day, the wind kicked up, the his two-man canoe capsized 300 metres from the lake shore and Tompkins was pitched into near-freezing water with 6ft waves. A helicopter eventually plucked him out and rushed him to hospital 75 miles away, but he died of hypothermia. “It took me five hours to get there,” says McDivitt Tompkins quietly.

“Going to Chile was a leap of faith. I retired at 43, I fell in love. I dreamed of going to Patagonia and living there. Doug brought the possibility to my mind – it was his idea to go. When I first moved down there, he was preparing to buy land for conservation. That was 1993. There was no internet, no [mobile] phones.

Her loss is immense, she says. “I was, I am, madly in love with him until the day he died. Every day of our lives. He made a 300-page book of our life together, thousands of pictures and letters. That life is over. Or not. I have had a charmed life, but I miss him so.”

Kris with Douglas in 2009. Photograph: Daniel Garcia/AFP/Getty Images

It shows, but Conservacion Patagonica and Conservation Land Trust, the two organisations the couple founded and which McDivitt Tompkins now runs, have been busy since the accident, and the work to restore and give back the land is now nearly complete.

Within a year, five new national parks in some of the wildest and most dramatic lands of Chile and Argentina will have been created, and three more will have been greatly expanded. With the two governments chipping in more land, more than 10m acres (over 15 times the size of the Lake Distric) will have been protected from logging, ranching, mining, hydro schemes and industrial development. “In a sense we were growing national parks,” she says. “They are like children.”

McDivitt Tompkins and her team these days are welcomed by the presidents of Chile and Argentina, but when the gringo couple first went to Latin America, their motives were questioned. “We were opposed for four years. We were ‘the couple who cut Chile in half’. They said we were setting up a nuclear-waste dump or a new Jewish state,” she says.

Leftwing nationalists accused them of American imperialism and neo-colonialism, rightwingers of wanting to steal Chile’s fresh water and of setting up a green-conservation cartel aimed to stop development. When they became one of Chile’s largest landowners, there were fears for national security.

“In Santiago, I was assured that Doug Tompkins was a CIA agent,” said Peruvian Nobel prize-winning writer Mario Vargas Llosa, who visited them. Chile threatened to strip the couple of their land holdings but when the threats of massive hydro dams, mines, and deforestation became clearer, people began to better understand their motives.

“The land we had bought first was owned by Chileans, mostly absent landlords. We bought farms and restored them. They ranged from Patagonian grasslands to forests and mountains. It was always the plan to give it back. We gave the last of the Argentina land back on 23 September,” she says.

They applied the skills they used to build their business empires. “We just rolled up our sleeves and did it. There is nothing we wanted to do that we didn’t. That’s not to say there’s not more to do.

A herd of guanacos - related to the llamas – in souther Chile in the region the couple created Patagonia National Park Photograph: Melanie Stetson Freeman/Christian Science Monitor/Getty Images

“If we hadn’t bought the land? Then the grasslands of Patagonia would not have begun to heal themselves, the rainforests would have been logged. We look at conservation as offering an economic development. I say we are not taking land out of production but bringing in something else. We employ hundreds of people and instead of kids leaving the area, there’s a reason for them to stay. There’s an economy they can plug into.”

Her model for conservation in the Patagonia region has been the US national-park system with its emphasis on tourism, walking trails, camping grounds and places to stay. But the conservation brief has expanded to returning animals to areas where they were hunted out. This brings her into conflict with farmers, fishermen and others.

“Rewilding is now a term used to bring back species. It’s tough. There is opposition from people; there may not be enough prey for the animals who are reintroduced. It is complex. We are now rewilding in northern Argentina and have brought back species such as giant anteaters, white-collared peccaries and the pampas deer. If you don’t have wildlife in these places, people will not visit. Landscape without wildlife is just scenery.”

The world is in poor shape, she says. “Can any system cope with the vast numbers of people we have? With huge population numbers ... it’s a perfect storm. I think it will take some kind of crisis before we are forced to take action as humans. It’s already a crisis. If people don’t call it a crisis now what are they waiting for?”

At heart, McDivitt Tompkins may still be the no-nonsense CEO, and is still a board member at the Patagonia clothing company. “It’s not that politicians cannot do anything but they are so focused on themselves that they can’t do the right thing. People need to get out of bed every day and have to do something that is not to do with them but to fight for something they love and identify with.

“We don’t have the luxury of being complacent. So, no more getting up in the morning and reading the Guardian and calling it a day. You have to act at the end of the day; you have to do something.”

“It’s not about money. There is just no excuse for doing nothing. Abdication is not a possibility. Whoever you are, wherever your interest lies, whatever you’ve fallen in love with, you get out of bed every morning and you do something. You act, you step into the fray, and you fight for a human society that is in balance with the natural world.

“We have no choice. Otherwise we might as well kiss our beautiful planet goodbye. I will be there in Patagonia till the day I die. That’s the idea.”