He seemed to blanch at the prospect of giving up more than two years of his life to the cause, while his friends made clear he was unmovable on the subject.
Yet on Monday, the former chancellor will have the nation's eyes on him when he launches the Better Together campaign from the campus of Edinburgh Napier University.
He said: "I decided that if I really believe in something, then I've got to be prepared to devote the time that I need for it. I don't think I could in all conscience have stood aside and said 'I'll leave it to somebody else'.
"I really do care very much about the future of the country in which I live. We are better together and it's something that I'm prepared to devote the time that's needed to it."
There was a time when the MP for Edinburgh South West was described as the most boring politician in Britain. Indeed, in 2002, when the beleaguered Stephen Byers resigned as Transport Secretary, Mr Darling was brought in to "take the department out of the headlines". He succeeded, much to the disappointment of Westminster journalists.
Yet it was his coolness under fire at the Treasury when the banking crisis struck that earned him plaudits. One aide memorably remarked: "The thing about Alistair is that the hairier things get, the calmer he becomes."
This quality is likely to come in handy as the heat of independence battle intensifies in the run-up to referendum day in autumn 2014.
It was interesting to note how, just days before the No campaign launch, the Nationalists unleashed an attack on Mr Darling, denouncing his "savage capital spending cuts" when chancellor.
In response, he had a warning for the SNP: "If this ends up as the Nationalists simply trying to play the man rather than the ball, they will be the losers ... People want to hear what the arguments are for and against. They are not interested in personal abuse.
"It's a sign of the Nationalists' weakness that whenever they are confronted with the merits of what they are proposing, they prefer to attack the individual rather than deal with the question in hand."
Of course, another line of attack by the SNP focuses on how the "Tory-led anti-independence campaign" will see Labour politicians standing shoulder-to-shoulder with Conservatives and Liberal Democrats.
However, Mr Darling insisted the temporary alliance would not damage his party. The public, he said, is "far more sophisticated than some politicians think" and could appreciate how on some issues they could come together to support a commonly-held view.
One possible advantage the SNP might have in the months and years to autumn 2014 is money. It is said to have more than £2 million tucked away in its war chest.
Yet when asked if the No campaign could match this, the former chancellor confidently replied: "Yes. We've already had pledges from a number of people. Campaigns cost money. We need to raise that money to fight a proper campaign and we will do it."
The Nationalists have called for all donations from outwith Scotland to be banned but Mr Darling brushed this suggestion aside, saying the SNP was "a party that has accepted donations from the United States of America, which is at least 3000 miles away. We're not accepting donations from abroad.
"But we don't regard England, Wales or Northern Ireland as foreign countries; they're not."
On the key constitutional question yet to be resolved – one or two questions on the ballot paper – the No campaign leader is determined to ensure there is just one.
"I don't think there should be any confusion about what we are deciding, especially as Alex Salmond said if there was a second question and it got 99% of the vote and independence got 51%, he would still claim victory, which shows he is playing fast and loose with the people of Scotland."
Dismissing the proposed devo-max question as a "cheap gimmick", Mr Darling said: "The only reason he is putting it forward is because he's frightened of the consequences of putting what he does believe in in the hands of the Scottish people. It would be a sign on his part that he doesn't feel confident in his own argument."
The former chancellor insisted the SNP bandwagon had "stalled", as evidenced by the recent local elections, and that polls showed the First Minister "does not speak for Scotland" on independence.
"That's why they resort to branding all their opponents in such offensive terms," he said, noting: "I do find offensive this idea that if you disagree with Alex Salmond, you're unpatriotic."
Mr Darling accused the First Minister of fudging independence "by refusing to use the word, by telling people everything will change but nothing will change and saying you'll still be British".
The No campaign is hoping Mr Darling's Treasury background will earn him respect among voters, particularly on the key area of economics where the battle for independence is likely to be won and lost.
The former chancellor is scathing about the SNP plans for an independent Scotland joining a currency union with England, as it would inevitably lead to political and economic union – in other words, Scotland rejoining the Union.
"You don't have to imagine how a currency union works, all you have to do is switch on the television every night to see how it works ... ultimately you're coming to political union.
"This is absolute madness. A referendum, five years of negotiation and you set yourself on a course that will bring you back to where you started out from."
Despite the polls and what he regards as the Nationalists in full retreat, the unflappable backbencher is not getting carried away.
"Yes, it's clear we are ahead at the moment ... but no-one can take anything for granted. We have to fight for every single vote and win the arguments at every turn. This is the biggest decision we as a country will take. It will have profound consequences if we decide to go. We can win it."
Mr Darling will launch the Better Together campaign alongside Annabel Goldie, the former leader of the Scottish Conservatives, and Charles Kennedy, the former Liberal Democrat leader.
Campaign sources made clear they would "not be aping" the launch of the Yes Scotland campaign, which many observers described as flat.
While there will be one or two celebrities attending, the emphasis will be not be on famous faces but on showing the breadth of support for the United Kingdom from ordinary Scots.