He managed to walk almost all the way from Croydon's King's Wood to High Barnet under tree cover, encountering grass snakes, woodpeckers, and a 2,000 year-old tree - but not one child.
From the heather-haired mountains of Scotland's Cairngorms to the chalk cliffs and vineyards of the South Downs, Britain's National Parks contain myriad natural terrain.
Plenty of variety, you might say, to satisfy most wilderness sensibilities.
But a campaign headed by a former geography teacher believes this trove of 15 special places, which preserve some of Britain's most stunning countryside, is missing one important landscape altogether, and is doggedly determined to put it right.
London, it argues, should be the country's first National Park City. London: with its 8.3 million inhabitants, and all the nitrogen dioxide they breathe.
It's a hard sell. As David Butterworth, chief executive of Yorkshire Dales National Park Authority, remarked on hearing campaign leader Daniel Raven-Ellison's pitch for the city:
"He is bonkers?"
Breaking the mould
Butterworth has a point. Philosophically National Parks are the antithesis of London, created in 1949 as a salve for urbanites evermore confined by bricks, mortar and industrialisation.
And 65 years later, as the majority of visitors to National Parks - such as the Peak District and Snowdonia - hail from London and the South East the city clearly still feels like a place to escape.
But as Raven-Ellison and I ramble the seven miles from London's Richmond Park to Wimbledon Common, under vast blue skies, the capital feels more kindred in spirit to the wilds of Exmoor than to its image as an oppressive, grimy dystopia.
And while the 34-year old explains the virtues of his campaign against a backdrop of rippling rivers, clumps of ancient woodland and sweeping fleets of deer, it sounds like nothing but logic.
It is the potential of 'green lungs' like these to remedy the ugliness of city life - plus a dose of Raven-Ellison's 'boldness' - that finally won Butterworth over. Declaring his backing in an official letter of support, the Yorkshire Dales chief wrote:
"Better biodiversity and wildlife. Increased opportunities for tackling obesity. Increased opportunities to create more volunteers and develop green space for our children. Why shouldn't London become a National Park City?"
Many others agree. A handful of senior National Park staff are expected to publicly express their support for the campaign, which has been running since April.
While the London Wildlife Trust, John Muir Trust, Guide Association, Friends of London Parks and Black Environment Network are already behind the campaign. Over a thousand have also signed the online petition to Mayor Boris Johnson.
Raven-Ellison - who left mainstream teaching so he could "educate on a wider scale" - is adept at persuading others to think laterally.
Selected as a National Geographic Emerging Explorer in 2012 for his work in pushing the boundaries of discovery, adventure and problem solving, his "Guerrilla Geography" excursions challenge children and adults to see place as more than a dot on a map.
Among his latest projects is a mission to walk the height of Everest by climbing London's tallest buildings - proving that you needn't travel far from home to find adventure.
Potential before beauty
The National Park City campaign - inspired by an expedition to all 15 National Parks with his 11-year old son - isn't suggesting the city's aesthetics are equal in worth to that of, say, the Brecon Beacons. Rather, it majors on vision and potential.
A Greater London National Park, Raven-Ellison believes, would capture public imagination, put the city's environment to better use and nurture "softer, more empathetic" relationships between people and their surroundings.
"If this was 1949 it might be appropriate to value a landscape owing to its beauty. But today we should be thinking about what is ecologically valuable. This campaign is about making a city more liveable."
Green space and water cover two thirds of the city, a wealthy natural environment that hosts richly textured plant, animal and insect communities. Fifteen hundred species of wild plant, 300 species of wild bird and 36 Sites of Special Scientific Interest are contained within its bounds, according to Natural England.
Yet, as Raven-Ellison outlines, "mental health conditions cost London £26bn a year. Obesity costs £900m. And one in five of the capital's children is overweight."
Greenery protects the public purse
National Park City status could help deflate those costs, he believes - though exactly how much he's not had the means to quantify.
Branded in this way families would be more aware of and make better use of London's natural environment. In turn, this would make them happier and healthier.
"The way people see cities is often through the lens of the media or politicians or a guide book. A National Park City changes expectations about the city and for the city."
This status, argues Raven-Ellison, could tackle the fact that on a recent walk from Croydon's King's Wood (south London) to High Barnet (in the north) he managed to walk almost all of the route under tree cover, encountering grass snakes, woodpeckers, and a 2,000 year-old tree - but not one child.
"I didn't even hear any. And it was a warm Friday during half term."
The experience, he says, is indicative, citing a 2011 London Sustainable Development Commission report that said one in seven London children hadn't visited a park with their parents in the previous year.
It could also help address the fact that in areas of London with sky-high rates of depression and suicide, green places happen to be poor quality, poorly managed or fenced off - a situation he came across on another exploration.
"If children aren't using the woods, then what impact will that have? National Park City status would create strategies to allow London's natural environment to evolve."
Answering the naysayers
To the critics who say London already gets too much investment he says that all of our National Parks would benefit: the city's special branding would alert tourists to their existence. He feels the same about London's outer boroughs.
"We have the new wetlands in Walthamstow, or the Colne Valley and Sydenham Woods. But if you were new to London you wouldn't necessarily visit these places because you'd be unaware of them. National Park City status could change that."
And to those who say it has been tried before and failed? "That isn't a good enough reason not to try again."
Less well-know destinations would be promoted through a flagship educational centre in Temple, at the heart of London, funded by the Greater London Authority.
It would also house a small team of staff to "inform, inspire, and co-ordinate best practice" in biodiversity projects and recreational activities, and serve as HQ for park rangers across the 33 boroughs.
A volunteer service, open to anyone from the age of three, would undertake litter picking and wildlife recording tasks and allow its 'helpers' to take part in an award scheme, such as the one run by John Muir Trust.
"It wouldn't have to cost a huge amount of money because there are already tens of thousands of people already delivering environmental services. We just need a great team to pull it all together, create new opportunities and unlock London's potential."
With or without Boris
Boris Johnson is thus far unmoved. In a letter to the campaign he explained that while the concept was "an engaging way of sparking debate" he doesn't have the power to create a new class of urban park.
Technically that's correct (Natural England which oversees National Park status) but Raven-Ellison believes he does have the means to get London's special stamp: "Boris is wrong. A mayor does not need permission to do this because National Park Cities don't yet exist."
"Actually I thought the letter was encouraging", says Raven-Ellison, who's already trying to win support of candidates for the London 2016 mayoral election. "Those standing for mayor need to recognise that the city could be at the forefront of a green revolution. It is a no brainer."
Current legislation precludes London's acceptance to the National Park family, with designated parks being defined as "extensive tracts of country with natural beauty and opportunities for open-air recreation".
Even cutting across the expansive commons of South West London that's a hard mental link to make.
'The needs are too pressing to ignore'
But Raven-Ellison, who's running the campaign from his kitchen table, thinks new criteria should be drawn up that reflect the green potential of a place and promote its liveability - a need he feels is made ever more urgent as more of us move to urban landscapes by the day.
"The issues are too pressing to ignore. And this is a big vision we can all get behind. Natural England needs to come up with a framework for cities. And if they don't we could just go ahead and declare London a National Park City ourselves anyway. We could just do it."
Perhaps Raven-Ellison really is bonkers. But does it mean he's wrong?
Learn more about plans for the Greater London National Park: join the 'Reimagining London' event on 24th February 2015 from 10.30am to 16.30. Tickets are on sale now.
Become a friend of the campaign.