'We're on the verge of independence ... this is the time of my political life'

The sheer effort going into the campaign, now and over the past 12 months, is staggering. Seen at close quarters, some believe the relentless round of town hall meetings and media appearances, not to mention the small matter of running the country, is beginning to take its toll. With three months still to go, it has been suggested one or two senior Scottish Government ministers are already looking exhausted.

That certainly cannot be said of John Swinney, Cabinet Secretary for Finance and Sustainable Growth. Sitting in his fourth floor office in the Scottish Parliament he appears lean, fit and, perhaps most striking of all, relaxed. It's the result, he says, of a new running regime that began small with a 5k race a few years ago and has developed into a serious habit.

Last year he ran the Aberfeldy half marathon (quite a step up from handing out medals the year before) after "locking himself in", as he puts it, by raising money for MS, the disease his wife Elizabeth Quigley, the BBC journalist, lives with.

With his 50th birthday on the horizon his success inspired him to attempt a full marathon, an ambition he achieved in Rome in March. Ever the politician, he said happily that his time of 4hrs 47mins compares favourably with the marathon running efforts of Labour's shadow chancellor, Ed Balls.

Now four stone lighter than when he first laced up his trainers, he notes: "Consistently over last four years I've felt my general wellbeing improving and my ability to cope with the volume of work that's got to be done and all the demands and rigours."

That has to be good news for the SNP. Alex Salmond and Nicola Sturgeon may be front and centre as the party's main referendum campaign "faces" but Mr Swinney, who led the Nationalists from 2000 to 2004, has a huge task on his hands.

"My role," he says, "is to be one of the key communicators in the referendum campaign and one of the key campaigners.

"There is a whole variety of debate and contribution in which I'll take forward what I'm talking about."

What he's talking about, of course, is the economic case for independence. The prospects for an independent Scotland have proved a bloody political battleground over the past year. The Scottish Government has been accused of basing its plans for improved welfare and pension provision, if Scots vote Yes, on over-optimistic forecasts for North Sea oil revenues. Respected think tanks, meanwhile, have argued an independent Scotland would start life deeper in the red than the rest of the UK and would face a tougher time balancing the books in the decades to come as oil declines and the population ages.

But Mr Swinney, who believes the economic debate is "fundamental to the argument and the outcome of the referendum", argues the unfavourable scenarios outlined by academics fail to factor in the whole point of independence: the opportunity it would bring to improve Scotland's economic performance. The Scottish Government believes tax revenues can be increased by £5billion by 2030 as a result of improved productivity, higher levels of employment and increased immigration. There can be no guarantees this would actually happen, but Mr Swinney insists the goals are "tangible".

He says: "The question members of the public will wrestle with is where does my best economic future lie?

"The judgement will be based on a whole variety of considerations.

"People can look at the United Kingdom and look at the position today and say, well, staying in the United Kingdom does not give me much confidence. We are living in this economic environment of austerity, where there is an acceptance across the political spectrum in the UK where essentially the offering to members of the public on the things we can depend upon diminishes."

He adds: "What the debate crystalises into is, can people see how economic powers could be used to create a more prosperous and, as a consequence, a fairer Scotland?

"That's where I think the debate will turn in people's minds."

To the dismay of many observers, both sides in the debate have attempted to distil their economic arguments into stark, bottom line numbers. Alex Salmond claims Scots would reap an "independence bonus" worth £1000 per person per year, based on the Scottish Government's forecast of £5billion extra revenues by 2030. Danny Alexander, the Lib Dem Chief Secretary to the Treasury, claims being in the UK is worth £1400 per year to Scots, his so-called "Union dividend".

"I don't think it turns on those numbers," says Mr Swinney. "I think it turns on a sense of economic wellbeing and what we will be arguing is that we've seen what Westminster has delivered and we can look into the crystal ball of that economic profile of the UK and you are looking through a prism of austerity.

"What we have set out is an approach focused on using the inherent talents, strengths and capabilities of the people of Scotland to realise our full potential.

"Ultimately, I think people will make a judgement about where they think their economic wellbeing in the widest possible sense lies, not just about a crude number about how much will I be better or worse off but about the economic wellbeing of the country."

Here Mr Swinney's vision extends well beyond the sphere of macro-economics and the usual questions of tax and spending. Influenced more and more, he says, by the growing number of theorists making the case that inequality causes grave damage to economies and societies, he believes independence would allow Scotland to tackle health and social problems alongside efforts to encourage the creation of wealth.

The example he gives is welfare reform which, he claims, is hindering rather than helping people fulfil their potential, to the detriment of the economy. He says: "We are encountering example after example where good work in our third sector is being contradicted by the impact of welfare reform. What I want is the ability to sit here and not explain how difficult the situation is but to resolve it."

Mr Swinney's thinking is shared by many on the left. He has little time, though, for Labour's argument that such issues are better tackled across a bigger, more economically resilient UK. Dismissing the idea that Scots are better off pooling and sharing risks and resources with the rest of the UK, he says: "Between 1997 and 2007 Labour had fabulous years of public finances, fabulous years of political domination and control and they never tackled the fundamental problems of our society.

"So we've had an opportunity to see what that could have delivered to transform Scotland and it didn't do any of it."

He also brushes aside the suggestion his leaked cabinet briefing paper, which dates back to 2012 and recognised an independent Scotland would face pressure on its public finances and "significant" start-up costs, had damaged the Yes campaign.

"The cabinet paper was essentially setting out a programme of work we would have to do to get ourselves ready to argue this case. That's what we did," he says.

The results, he says, are to be found in the Scottish Government's independence White Paper, Scotland's Future.

The MSP for Perthshire North, and former MP for Tayside North, is convinced his arguments will prevail on September 18.

There is no need to ask whether he believes the Yes campaign will win.

Just how it might achieve victory, though, with the polls consistently showing a lead for No, is a question he cannot avoid.

"By doing two things," he says. "By providing a hopeful, ambitious and positive message to the people of Scotland and secondly by energising the public through active and positive grassroots campaigning.

"I've been involved in campaigning for 35 years and I thought the level, tenor and energy of our 2011 election campaign would be difficult to surpass. I'm in awe of what's happening now. Every election I fight new people come on the scene in the constituency. For four weeks of the election campaign new faces appear and they are very welcome, they get involved and that's great.

"For the past 12 months it's just gone on and on and on. People who we have never dealt with as political campaigners pitch up and say: 'What can I do to help?' When I look out now at the teams of folk we have out and about, working away in all sorts of ways, actively arguing the case and not just formal campaigning, it is quite unbelievable."

This, surely, is one reason Mr Swinney is relishing the referendum so much and why, if the campaign has been a marathon, he looks like a man with a sprint finish in him. He loves the party he joined at joined at 15 and the party loves him back.

The grassroots might respect him as "Honest John", the solid, reassuringly capable minister, they might admire his persuasive oratory and forceful debating style, but above all they recognise and revere the deep passion for independence that burns within him.

He says: "I came into Scottish politics to secure an independent Scotland. That's what motivated me to come into politics and here we are.

"We are on the verge of it. It's the time of my professional life. I'm loving it. It is the fulfilment of my political involvement."

It was not, as some profiles have suggested, a sports commentator's slight against swimmer David Wilkie that first nudged the young Swinney towards the cause. Rather, like many of his generation, he was politicised by the presence of nuclear weapons on the Clyde.

He recalls: "What got me into politics was that I was motivated in the late 1970s by a desire to eradicate nuclear weapons from Scottish society.

"As a 15-year-old schoolboy I thought it was morally offensive that we possessed nuclear weapons and wanted to do something about it.

"It became clear to me Scottish independence was the only vehicle that was going to ensure that was achieved, which is why I joined the SNP. That's where my political motivation came from."

At first his activism and efforts to build a career in politics surprised his motor mechanic father and medical receptionist mother, neither of whom were political when he was growing up.

"I think they saw it coming when they saw how much I wished to do something about the nuclear issue," he says.

"Wishing to do something about it rather captures what I'm about.

"In a sense it's relevant to the debate. I look at a lot of the stuff that gets kicked about in the chamber downstairs and, for example, our Labour counterparts will bemoan things that are happening within the economy or within society but don't want to acquire the powers to do something about it. I just don't understand that."

Mr Swinney's office is noticeably Spartan, in contrast to that of his boss where a vast Gerard Burns painting, The Rowan, hangs behind the First Ministerial desk.

The only decoration is a small stained glass window, picking out the SNP party logo in shades of blue. It was presented to him by the artist a couple of days after he was elected as an MP in 1997.

Why this, of all the mementos he could choose? The inscription says it all: "Now is the day and now is the hour."