HIGH above the cobbles of one of Edinburgh's grandest streets, Alistair Carmichael is in his garret office, lolling on a natty striped sofa.

The man fronting the Coalition's defence of the Union north of the Border was dubbed a bruiser and a Rottweiler on becoming Scottish Secretary in October, but up close he looks more like one of the seals from his Orkney and Shetland seat as it dozes contentedly after lunch.

The fate of the United Kingdom may be on a knife-edge, but there's apparently no need to panic at the Scotland Office. At his Christmas reception, Carmichael said the run-up to the independence referendum would be the most challenging spell of his career, so how is he preparing and psyching himself up?

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"You want to know what my gym routine is?" he laughs. "I've been an MP 12-and-a-half years. It's not always regarded as a compliment, but I'm a professional politician and as a professional politician you know what you're signed up for.

"I think it will not be that much different from the normal day-to-day political campaigning, except that there will be an intensity about it. This must be a once-in-a-lifetime choice. Please God we're not back here in 15 years' time. If Scotland votes Yes, then there's no going back. A Yes vote is a one-way ratchet."

Not too intense a campaign, mind you. He plans to stick to his current travel routine, returning to the Northern Isles every weekend to be with his family - he has two sons, aged 12 and 16.

"That's important to me, that's the sanity valve. As long as I can just spend some time mucking about with my boys, or walking the dog on the beach," he says.

But can he really keep that up to September 18? "That's my intention. Look, this is an important, exciting job, but I have other jobs as well. I'm still the MP for Orkney and Shetland. I would never want people in the Northern Isles to think that I was too busy to do the job they elected me to do."

It may not be his job for long, however. Unusually for a serving MP, Carmichael admits he is already pondering a new career.

He intends to stand for re-election in 2015, but beyond that he may quit politics altogether. After hotel management and the law, he considers "career politician" to be his third career, and thinks there is a fourth in him as well. Better that than becoming stale, like some others he could name at Westminster.

"I think there is a fourth career," he says. "I don't know what it would be yet. But there is always a risk for every politician that you stay too long, that your shelf life has past. At the end of that [2015] parliament I will be 54, so maybe, at that point, I might want to look at something else."

Given the start to his new job, he could be forgiven for ruminating on pastures new. Although respected in the backroom role of Liberal Democrat chief whip at Westminster, his shift to front-of-house politics was a painful one.

Within weeks, Carmichael was thrust into a TV debate with the SNP's Nicola Sturgeon. As viewers peered through their fingers, he was subjected to one of the bloodiest political clubbings of recent years, his unanswered pleas to the adjudicator to restrain Sturgeon's ferocity only adding to the pity of it.

He is still beating himself up over it - but not too strenuously, and says it has to be set against the success of other activity, such as working to secure jobs at the Grangemouth petrochemical plant and at BAE on the Clyde.

He says: "It wasn't the greatest performance I've ever put in, I fully accept that. It annoys me a bit because I didn't do as well as I can do.

"I know that next time I'm going to have to do better, yup. But, you know, it was one half-hour session out of nine, 10 weeks in the job. I'm not going to let one less-than-great performance define my job."

What will define his job is the referendum. To help achieve a No vote, he says the Unionist side will soon be stepping up a gear.

Although the recent Scotland Analysis papers have provided "real grounding" and intellectual rigour to the case for retaining the UK, he acknowledges they have been rather dry, and a more "accessible" message is needed for voters.

In practice, that means boiling down most aspects of the independence debate to jobs. "Take something, for example, like the debate on currency. It matters because you have to have a stable currency in order to allow businesses to grow, in order for people to have jobs.

"You have to be able to link the currency to jobs, because that's the point at which people will sit up and take notice and say, 'Ah, that's why currency matters'. It's not about whether I have to change into euros or dollars or yen when I go on holiday, it matters because it means that I'm more likely or not to have a job. It's what's going to put bread and butter on the table that really matters to most people."

Dismissing SNP criticism of Better Together as negative and lacking in vision, he recounts giving a recent talk to business professionals and being asked for his vision for the UK. "I just pointed to the slide that was on the screen and said, 'There's my vision'.

"We have just come through one of the worst economic shocks in history and we are now part of the fastest growing economy in the G7. As a result of that, there are 11,000 people this month who've got a job who didn't have it last month. That's a pretty compelling vision to my mind. And for the 11,000 people… because we're part of a successful growing economy, I think that means more to them than some illusory promise on childcare [from the SNP]."

Carmichael and Sturgeon are a textbook example of the difference between a job and a vocation. Carmichael is performing a function; Sturgeon is driven to the point of compulsion.

She'd be "gutted" to lose in September; he doesn't think it would mean "anything more personally … than it does for any other Scot".

It's Joe Bloggs versus the Terminator. Surely he is at a major disadvantage up against the Yes side's all-consuming zeal?

But again he is unfazed. "I would say it's an advantage," he says. "There are only a handful of people in Scotland today who obsess about the constitutional issue in the way that Nationalists do. The vast majority look at this and don't quite see why it is so important, why it is such an obsession, why it's worth putting the government on hold.

"My concerns are principally about jobs for people that I represent, for the education and the future of my children. I think that puts me more in tune with the rest of Scotland. Very few people will say the most important thing to them is whether Scotland is independent or not."

Joe Bloggs isn't out the game yet.