I met John Swinney, Scotland’s deputy first minister, this week for the first time in what must be more than a decade. We’re both older, obviously, but while I’ve begun to sag and creak, Swinney remains depressingly taut and angular.

His impressive physical condition, and evident good health, was matched by a chilled demeanour and relaxed sense of humour, and it all made me think: how on earth has he managed it? Swinney has been a senior cabinet minister for 11 years, and was SNP leader for a few years before that.

And then I thought about his boss, who has also been at the top levels of government since the SNP first took power in 2007. Nicola Sturgeon shows equally little sign of wear and seems to cope with the extreme pressures and hours of high office without feeling the strain over much. Look at the enervated state of Tony Blair these days. Remember what power did to Gordon Brown.

I recall an aide to Ruth Davidson describing the nationalist leaders admiringly as “relentless. They just keep going.” And that’s another thing: Sturgeon and Swinney haven’t just been running a government and a party, they’ve been running a cause. In this decade alone they’ve been through three referendum campaigns, three general elections and two devolved elections, and numerous day-to-day crises. The question must be put: why aren’t they completely knackered? Shouldn’t they be burnt-out, shambling shells by now?

I’m afraid I was unsubtle enough to put this directly to the Deputy First Minister. He laughed and asked me why I thought it was. I said something like “it’s because you’re not finished yet – being in government isn’t the endgame for you. You won’t rest until you’ve secured independence.”

That was part of it, he admitted, but there was something more important. In all his years running the finance department and now education, he had never had to worry about friendly fire from his own side. He had never felt briefed against, or undermined by ministerial colleagues. The handover of power from Alex Salmond to Sturgeon had been achieved with no challenge or fuss. He was able every day to come in and get on with the day job. Compare that to the fratricidal nature of Westminster politics at the moment, or to the fraught TB-GBs outlined in Alastair Campbell’s diaries and elsewhere, which regularly knocked off course what was otherwise a very good government. How much energy and headspace is consumed by constantly having to watch your back, never mind your front?

This esprit d’corps springs from necessity. When the SNP won the 2007 Scottish election, they were the largest party by just a single seat, with 47 to Labour’s 46. Alex Salmond impressed on his troops on the day after the election, before Holyrood had reconvened to confirm the new government, that the tiny margin of victory meant that if even one of them stepped out of line it could hand power back to Labour. The Queen’s signature on the warrant making Salmond First Minister was rushed through, and couriered up overnight on the Caledonian Sleeper, to avoid the risk of mishap by an errant MSP.

This discipline became a habit of mind. The SNP had always had more than its share of internal squabbles, especially between the “gradualists” and the “fundamentalists” who took different views of how best to secure independence, but they now fell into a new way of being. They did not brief against one another. They had their disagreements in private, but presented a united front in public. They didn’t use up valuable energy trying to take down one another, because they needed it all for the cause.

There is a downside to all this for Scotland, because, like every other party, the SNP is a broad coalition. It has been hard for journalists to get behind the scenes and establish what’s going on beyond the spin. There have been accusations of Moonie-like behaviour and of always putting independence before the natural and healthy process of public debate.

But regardless, it has served the SNP well. That their poll rating still sits around 40 per cent after so long in office is remarkable. Public trust in the devolved government seems to remain equally robust. The chasms opening up in Labour and the Tories only serve to highlight SNP unity. And as Westminster wobbles on its axis, the Scottish government looks more than a little boring. In a good way.

It’s a fascinating psychological study of a different way of doing politics. That public “band of brothers and sisters” attitude has served the Nats well. It certainly seems to help with the stress levels. Perhaps the other parties, if they survive, might think about giving it a go.