There was, of course, no chance that Prince Philip would take things easy. Putting his feet up after retirement was never likely to be his style.
It is why, most Mondays in the run-up to Christmas, he could be seen, reins in hand, carriage-driving in the grounds of Windsor Castle or, on other days, motoring around the estate to deliver festive greetings to staff.
Even so there cannot be many men in the land taking on a new enterprise well into their 98th year.
But this week it emerged that after 12 fruitless years trying to coax black truffles from the soil at Sandringham, Philip has become the first person in Britain to cultivate a successful crop. This has been more than a labour of love.
Ever since he was introduced to the eccentric pursuit of truffle-hunting by his uncle Earl Mountbatten of Burma on a trip to Italy, Philip has been fascinated by the idea of growing the delicacies himself.
Duke of Edinburgh has finally succeeded in growing a crop of black truffles after 12 years of trying
He began the exercise in 2006 when he planted more than 300 saplings of hazel and oak impregnated with truffle spores at £15 apiece.
He knew that the royal estate in Norfolk was well suited for the fungi because of its abundance of alkaline soil.
His attempts to create a truffiere in the Royal Fruit Farm, which yields apples and gooseberries as well as having a 100-acre blackcurrant plot — much of whose crop is destined for Ribena — had become something of an in-house joke, as year after year truffle-sniffing dogs failed to root out a thing.
Four years into his experiment, Philip drafted in Italian experts, who warned that the first crop might not be harvested until 2021, the year of Philip's 100th birthday.
A basket of Perigord black truffles pictured at a French market. Prince Philip now has his own crop of the delicacies
Their pessimism was misplaced. And the plantation, tucked away in the Organic Zone on the Sandringham Estate, is proving the doubters wrong.
The black truffles it is suddenly producing are so prized by gourmands that they are nicknamed 'black diamonds' and are valued at very nearly £1,000 per kilogram. They also have a reputation as an aphrodisiac.
Adrian Cole, a director of Truffle UK, which supplied the Duke of Edinburgh's trees, said he believed Sandringham was the first place to harvest black truffles in the UK.
The truffles, known as 'black diamonds' can change hands for as much as £200 for 3.5oz
'The majority have been Tuber melanosporum: the French Perigord black truffle — as good as you can get,' he said.
The plan had been to sell the crop to raise money for the running of the Estate, which has been managed by the Duke since her accession to the throne in 1952.
But Mr Cole said he understood the early crop had been consumed within the Royal Family. 'From what I gather, none have been sold. They have gone to the house or members of the family.'
Philip has become the first person in Britain to cultivate a successful crop
Truffles are an important component of some haute cuisine dishes in French and Italian cuisine
However, a peek at the Sandringham cafe menu suggests otherwise. For £7.95, visitors can sample a Sandringham game and Norfolk truffle terrine, served with pickled vegetables.
So what fired Philip's passion for a pastime which requires more than the usual store of patience and perseverance? And why should a man not known as an epicure want to grow such a pungent delicacy anyway?
The answer, it seems, was because he could not resist a challenge.
Since the early Sixties when he and Mountbatten visited Italy, where truffles are considered as luxurious as the finest caviar, Philip has been intrigued.
On some of these expeditions they are understood to have had a youthful companion, Sacha Phillips, later to become the Duchess of Abercorn and who died three weeks ago.
Many years later, the Earl's former aide John Barrett claimed in a memoir
