Pedalling on water for more than a hundred miles in a heatwave, pushed back by east winds and having to navigate 31 locks would be a challenge for anybody. But when that body is 90 years old, with a bad knee, failing balance and malfunctioning arms and shoulders, it’s a herculean feat.

Rainforest campaigner Robin Hanbury-Tenison, 90, is pedalling 104 miles down the River Thames from Oxford to Richmond on a water-bike to raise money for a unique research station which is being built to study Britain’s temperate rainforest.

Hanbury-Tenison, the founder of Survival International, who spent much of his younger years raising awareness of the value of tropical rainforests from the Amazon to Borneo, has turned his attention to overlooked temperate rainforest after discovering that his modest Cornish hill farm had an important fragment of the mostly vanished habitat.

“I’ve seen what we’ve lost. At my age, if I’m going to do something about it, I’d better get on with it,” said Hanbury-Tenison.

Starting on Friday and supported by his son, Merlin, who will ride alongside him, Hanbury-Tenison will navigate a pedal-powered craft that sits on the surface of the water from Magdalen Bridge, Oxford, aiming to finish at Teddington Lock in Richmond on International Rainforest Day on Monday.

Merlin said of his father: “As a 90-year-old he finds he has been slightly falling apart. He’s got a bad knee so he’s trying to rest that to get ready. He did a rowing challenge but found his arms and shoulders stopped working. He climbed to Cornwall’s highest point but his balance and his legs have gone, so he’s worked out he can sit on a bike and peddle. It’s like a rickety old car going down a mountain with bits falling off it.

“I’m going to take a stick so I can beat him when he slows down. It’s going to be a tough one. I just hope it doesn’t finish him off,” he added.

Hanbury-Tenison has been offered some support as he makes his way down the river. “A number of people living on the Thames have said: ‘Drop in for a cup of tea’. As if I have time for a cup of tea,” he said.

How are Hanbury-Tenison’s injuries? “I’m 90 for goodness sake. Of course things begin to hurt but one pedals through the pain threshold,” he said. “I’ve been doing lots of training. All I do is pedal away on an exercise bike. I’ve been tempting fate by saying what could go wrong – I’ve just got to pedal for a few hours each day.”

He hopes to raise £100,000 towards the construction of Europe’s first dedicated temperate rainforest research station on his farm on Bodmin Moor, Cornwall, by the Thousand Year Trust, the charity run by his son, Merlin.

Hanbury-Tenison said: “I’m proud of being part of the movement that showed the world that tropical rainforests matter, are endangered and need to be saved. To find that the scruffy piece of land on this small farm is a last vestige of the equally rich but even more endangered temperate rainforest is extraordinary.

“We’ve lost half of the Amazon in my lifetime, which is disastrous, but temperate rainforest that used to cover a fifth of Britain is now down to less than 1%”, he added. “If you want to study a rainforest, you can study at tropical rainforest research stations all over the world but there is nowhere you can study the temperate rainforest at home.”

The first phase of the interdisciplinary research station, constructed with locally sourced timber provided by the Woodland Trust, is under construction but the Thousand Year Trust needs to raise several hundred thousand more to complete the facility. The project has received support from more than 20 university research partnerships, including the universities of Exeter and Plymouth, and is described by the Thousand Year Trust as “a once-in-a-generation effort to save Britain’s rainforest”.

Hanbury-Tenison’s fundraiser has been boosted by actor Russell Crowe, who has pledged to match the first £25,000 raised to take the total to £50,000. “What a champion Robin is,” said Crowe.

As well as headwinds, locks and the ravages of time, Hanbury-Tenison’s final challenge will be the weekend’s heatwave. How will he cope? “I prefer heat to cold and I’ll wear a good floppy hat,” he said.