As the windows are wound down, everyone inside is laughing. The urge to cover up becomes almost uncontrollable but I am determined to tough it out. After shaking a few extended hands, they start to get out and take in turns to stand beside me and have their photographs taken before driving off, the laughter now ringing out louder than ever. Strangely, I don't feel ashamed. What I have is a sensation akin to pride. For someone who feels self-conscious wearing shorts in public, this feels weird.
Such was my first encounter with the clothed public when I joined former Royal Marine Steve Gough and his girlfriend Melanie Roberts and their 60lb rucksacks for a naked ramble in the Cairngorms.
Mr Gough has become something of a celebrity in the last two years since he decided to launch his campaign for the nascent liberty movement Freedom To Be Yourself by making the 847-mile unclothed journey from Land's End to John O'Groats. On that occasion he was arrested 15 times and spent 140 nights in prison. The police referred him to a psychiatrist while his ex-wife and mother lambasted him in the press. He eventually reached the northern tip of the United Kingdom in the perishing month of January, five months behind schedule.
Undeterred by his experiences, the father-of-two embarked on the same route again in June this year. This time however, he was accompanied by his new girlfriend Melanie, a hairdresser from Bournemouth, close to where Mr Gough, 46, spends a lot of the time parked up in the Ford Transit van in which he now lives. The couple met, naturally enough, on the naturist beach at Studland Bay two weeks before the planned departure. A veteran of naked sunbathing since she was 17, Ms Roberts, 34, only decided to join him on his latest odyssey at the very last moment. They were joined by a third naked walker, a retired librarian from Beckenham, Kent, who completed the walk in late August.
It has been a winding road since the sunny start line on the beach in Cornwall, surrounded by Page 3 girls courtesy of The Sun newspaper. After more than three months walking up to 25 miles a day in sun, wind and rain, their bodies are toned and they boast deep brown, all-over tans. And to speed them on their way they have been joined by dozens of supporters, some of them - like me - going naked for the first time. Some have spent their annual holidays walking with the couple. More than 10,000 people have logged on to the Freedom website to chart their progress.
But it has not been all sleeping under stars and wind in the hair. As well as suffering the official disapproval of British Naturism, which represents the UK's 17,000 naturists, for being part of an allegedly "confrontational" stunt, Mr Gough has been arrested on five occasions and spent three weeks in custody. Ms Roberts has been detained three times. Their collars were first felt - metaphorically - in We in Shropshire. A few weeks later they were arrested again just outside Skipton in North Yorkshire after shopping naked at a village store. On that occasion Mr Gough's refusal to cover up earned him a second spell in the jug from Harrogate magistrates.
Moving north of the border they came a cropper with the law again outside Edinburgh. Mr Gough was sentenced to two weeks in jail. He was re-arrested as he was released for refusing to get dressed. The couple's greatest test still lies ahead as they enter the territory of the Northern Constabulary - a force which proved public nudity's most implacable foe last time.
I joined the naked couple following their latest release from jail. They have spent the last three days enduring some atrocious Highland weather - bitterly cold winds and driving rain - as they made their way across the high peaks of the Cairngorms north of Pitlochry. Descending into the calm of the Spey Valley, the sun makes a merciful appearance, although as the wind drops and the temperature rises, so appear the dreaded midges.
Mr Gough admits that cracks are starting to appear in his philosophical armour. "When I first started doing this I thought I had reasons. I thought I knew why. But the more I go on I realise that there isn't a reason." He has grown used to the impact he creates, politely acknowledging each hoot and clap as we make our way along along the banks of Loch Morlich on the Aviemore road.
This time there has been no repeat of the attack at St Ives by a group of lads that left him badly beaten. The negative comments - apart from those of the magistrates that sentenced him - have been virtually non-existent. "The worst we have had so far was a man in a van who said "put your clothes on", he said.
Ms Roberts has not been harassed - apart from the occasional farmer who has driven his Land Rover around the block for a second, or third, look.
They both remain committed to the cause of public nudity, or more accurately the right to go naked. "People talk about a healthy self-image and that it is important to be comfortable with your own body. We are challenging people to think about how they really feel about their own bodies. People talk about freedom but I can be locked up for just walking from A to B naked," said Mr Gough shaking his head in exasperation.
There remains a minority that feel naked rambling should be punishable by law, which means the police must continue to act. But for Ms Roberts the equation is simple. "The overall effect is that it makes people happy. Everyone likes a naked body." I ponder her comments as Steve informs me that we are preparing to rendezvous with a BBC documentary crew making a programme about the walk.
As I see them waiting in the car in a lay-by at the side of the loch I conclude I have reached the limit of my desire to be publicly naked. For all my new found self-confidence the idea of my nakedness being captured for perpetuity on camera sends me diving for my clothes.
They climb into clothes for lunch - as they only remain naked for walking - and become just another young couple out for a walk in the woods.
So who has the problem? The people who feel so uncomfortable at the sight of two fine specimens of the humanity striding through the British countryside in the all together? Or is it them for flouting convention and challenging society's ancient taboos? How can the naked body really be shameful?
I cover up again and feel a sneaking admiration for their strange, epic journey. Will I be doing it again? My family is begging me not to and no matter how intellectually I reason that there is nothing wrong with being naked per se, for me it just feels too damn weird doing it in public.
Naturists' struggle to shed their bad image
Ever since the Earl of Mercia's wife ordered the people of Coventry to close their shutters while she rode naked through the streets to secure them a tax cut, public nudity has had a particular pull on the British psyche.
As Lady Godiva made her legendary progress in about AD1000 to force Leofric III, to abolish his tolls, a tailor supposedly disobeyed her proclamation and spied on her, only to be struck blind. His curiosity gave the English language the expression "Peeping Tom".
This trade-off between the principled shedding of clothes and the risk of exciting a prurient "phwoar" or "eeurgh" from the unenlightened has provided the nation with comedy, outrage and entertainment for the 1,000 years since. From the Carry On movies to Health and Efficiency magazine, and Lady Godiva to Erica Rowe, the 24-year-old bookshop assistant who became Britain's first female streaker at Twickenham in 1982, there are few acts more guaranteed to earn publicity in Britain than the simple fact of getting naked where others are not.
It is a response thatirks Britain's 25,000 signed-up naturists, along with the estimated 500,000 who regularly go nude.
British Naturism, a 16,000-strong nudists' organisation, has spent four decades trying to dispel the notion that nakedness is, of itself, anything to do with sexual attraction and is instead everything to do with a oneness with mother nature.
A spokesman said: "It is a way of life in harmony with nature with the intention of encouraging self-respect, respect for others and for the environment."
Some within the movement frown upon the militancy of Steve Gough, who first made headlines with his naked rambling in 2003, believing that it once more gives a laughable quality to a past-time trying hard to shed its image as an object of ridicule conducted largely by corpulent middle-aged couples on Leylandii-screened campsites.
Unsurprisingly, serious nudists also eschew streakers, the breed immortalised in popular history by Ms Rowe's topless sprint across the turf during the England-Australia rugby match. In the aftermath of Ms Rowe's 15 minutes of fame, one nudist organisation commented: "Exhibitionism is not nudism. Nudism is about people who enjoy being naked but are not motivated by seeing others naked or being seen naked. That is very different from baring your 40-inch chest in front of 50,000 people."
The history of British nudism is, ironically, a little hazy. The father of modern naturism is Richard Ungewitter, a German who published a book, Die Nacktheit or Nakedness, in 1903 explaining the pointlessness of clothing. He also espoused a Spartan regime of vegetarianism, compulsory gym exercise and being teetotal.
It was not until the 1920s that the movement made its way to the UK.
A nudist club opened in Wickford, Essex, in 1922 but it remained an activity only to be conducted on private land for almost 60 years. The first of Britain's small crop of designated nudist beaches opened in 1978 and there are now more the 150 naturist clubs across the country.
But rarely is the subject of nudism greeted with sober discussion or, as in Germany or the Netherlands, a disinterested shrug. When naturists at Studlands Bay in Dorset complained about the shrinking size of their nudist beach, they were greeted with stories about "naked fury".
Exponents of the birthday suit, however, can retort with words of one of the nation's foremost seekers of spiritual enlightenment. John Donne wrote in 1598: "Full nakedness! All joys are to thee."