What does a Catholic priest do upon relinquishing the trappings of parish ministry and retiring after 50 years' service?

If you are Father Willy Slavin, you eschew the farewell party and the retirement home, and set off in your Ford Transit camper van to devour 7000 miles of Scotland's coastline. He's already completed the Munros, has cycled from Land's End to John O'Groats and now he's hungry for more.

At 74, he's fit and healthy and is making the most of it while he can, not only as an active part of a generation he describes as "pioneers of old age", but also for the generations who will follow his, actively working into their 70s and not really knowing what to do in retirement. Buying a second home in Spain is not, he says, the most ethical solution to this modern-day conundrum.

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Freed from the routine of daily Mass and weekly hatches, matches and dispatches, he is basing his new life on Psalm 90, which includes the words "the days of our years are threescore and ten" and continues with the oft-unspoken coda: "they will be extended to 80 if we are strong".

Slavin, whose slim build is maintained through fasting as well as exercise, contemplates the implications of this lesson.

"God's promise to us is not that we'll live to 70, but that we will live healthily to 80, so we know that's what he wanted us to do," he says. "I went away because I was keen to gain a perspective on how I might use the extra 10 years as preparation for bringing my life to a close, and at the same time to reflect on what it's all been about.

"There are lots of healthy 70-somethings who don't know what to do. Most people live lives of quiet desperation."

Such existential angst might be shocking to hear from a Catholic priest, but then Slavin has never been afraid of speaking his mind, from the pulpit or elsewhere, in order to "discomfit the comfortable".

But despite his often abrupt manner, Slavin - who entered Blairs seminary, Aberdeen, at age 12 - is a most humble man, driven by the imperative to live by example.

A committed anti-capitalist, he devoted his life to helping those less fortunate, quietly and without fuss, long before Pope Francis re-introduced the world to the concept of humility, poverty and charity as the true Christian way of life.

He was the founding chairman of the homeless charity Emmaus Glasgow and the Scottish Drugs Forum and was chaplain of Barlinnie Prison for 10 years. Away from the gaze of his relatively affluent west end parishioners, he served in street cafes and continued to visit those serving life in prison. A qualified chartered educational psychologist, he was a consultant in alcohol and drugs to Notre Dame Child Guidance Clinics and chaplain to Queen Mother's Maternity and Yorkhill Sick Children's hospitals.

He doesn't wear a dog collar and has no truck with ostentation of any kind.

He laughs off comparisons with Pope Francis, who chose his name after the saint known for his devotion to the poor - though the Feast of St Francis turned out to be Slavin's official retirement date. What does he think about comments that the Holy Father is ushering in a new era of humility at the Vatican, based on the Catholic Liberation Theology (CLT) of the 1960s?

Slavin says: "Otherwise known as the Preferential Option for the Poor, CLT was never really followed through by the Second Vatican Council, certainly not by the Polish Pope John Paul II. Pope Francis is having another go at it."

The idea of researching healthy old age had been growing for the last few years of Slavin's ministry as parish priest of St Simon's in Glasgow's Partick and really took root when he turned 60 at the Millennium - the year he cycled from Edinburgh to Cologne for the Cancel the Debt protest at the G8 summit.

Fascinated that many people of his generation, who grew up in the post-war years of rationing, are the first to enjoy an active old age, he applied for a £7000 Fellowship to the Winston Churchill Memorial Trust and spent six weeks in the US studying the possibility of healthy ageing.

One of the outcomes of that experience was his conviction that, barring accidents, the NHS should be closed to everyone after they reach the age of 18.

"There's only health insurance in the US, so you have to look after yourself. I think that's correct. It's very important we learn to regard the NHS only as a last resort."

The devastating levels of obesity and ill-health he saw in the US were mostly down to poverty. "The problem is not health; it's inequality of access to positive health care," he says, echoing the sentiments of Sir Harry Burns who resigned from his job as chief medical officer for Scotland earlier this year.

Being outspoken has got him into trouble. He's only half-joking when he says that, at age 35, he went to Bangladesh to work for five years as a "reward" for preaching poverty and discomfiting wealthy west end parishioners.

"The archbishop said he would teach me a lesson by letting me work among the truly poor," he says. "In fact Bangladesh was the best thing that happened to me. Being face to face with abject destitution and poverty as a result of global consumerism was a turning point. It gave me the confidence and courage to say what I thought." He says that's why he's "a figure of suspicion" (although that's hard to believe since a memorial window dedicated to him is to be installed at St Simon's with £5000 donated by parishioners).

His overriding vow remains that of obedience to his bishop, so to describe him as a loose cannon isn't exactly fair, even if he did smuggle a louse-ridden pillow out of a cell at Barlinnie to show governors "the living conditions were worse at Bar-L than in Bangladesh". After telling the Church of Scotland he knew there were going to be roof protests in the infamous prison riots of 1987, his comments were reported to the Scottish Prison Service and Slavin was accused of whistleblowing and threatened with dismissal.

"I told them they had no jurisdiction over a Catholic priest and invited them to ring Cardinal Winning. They never did." He agrees with Lord Carloway, the Lord Justice Clerk, who said in January Scotland's prison population was too high and sentencing should shift from a culture of retribution.

Perhaps unsurprisingly, Slavin has little time for the Church "hierarchy" and was disappointed by revelations about Cardinal Keith O'Brien who resigned last year over allegations of inappropriate sexual behaviour. "His voice as leader of Scotland's Catholics is a loss to the referendum debate and to poverty issues."

While conceding ongoing problems within the Church, he is surprised at the recent accusation from the UN that the Vatican has failed to acknowledge the huge scale of clerical sex abuse.

"I have only one four-letter word to say about that - Eton," he says, referring to the single-sex boys' public school system, known as the "chief nurse of England's statesmen".

Asked about his own promise of chastity, he says when he was ordained in 1964 it was much easier to accept because "sex had only been invented in 1963" and nobody enquired about priestly celibacy. "We were left to get on with it as best we could. Now I think it's more difficult for young priests because society is sex-obssessed."

Eventually, Slavin wants to end up in a hut. He's a member of the One Thousand Huts movement which campaigns for the reforesting of Scotland and the release of more land for public use.

As he retreats to his trusty Transit, I ask if he's ever had doubts. "Perhaps I've lived at a low level of faith all my life, so I've never doubted it. It has however always been questioned. I really don't know any more than anybody else. My journey is ongoing."