This comment, in a prepared speech by Prince William in India this week, was a frank admission to an astonishing fact: Queen Elizabeth II is still actively running the royal franchise despite the fact she turns 90 on April 21 (her actual birthday, as opposed to the official birthday holiday in June).

She has reigned longer than any other British monarch, passing Queen Victoria's record last year.

She has visited more places and met more people than any other monarch, according to back-of-the-envelope calculations by those who count such things.

She is old. According to official statistics, the average life expectancy of a woman born in 1926 was 70 years.

At this stage, you'd think the average ruler would rest on their laurels a little. Certainly, Elizabeth II would be more than entitled to sit back in one of her many comfy armchairs with a gin or three and let the ever-growing number of direct heirs pick up the reins on her behalf.

Even ignoring the perennial rumours about an abdication (never going to happen, insiders say, flatly), two years ago there was speculation her health was no longer up to it. Royal watchers noted unsteady legs, a hand on Prince Philip for more than just reassurance on the steps of St Paul's.

And last November on a visit to Malta, her home for a short carefree time with her new husband more than 60 years ago, there was widespread reporting that it could be her last visit. She "seemed to be saying goodbye", extrapolated the Telegraph's reporter, as the local brass band played Thanks For the Memory.

But the Queen will clearly have none of it. She may be cutting back on long-distance travel but she is still seen riding her horses around the royal estates – and she is, as William said, still very much the boss of the family outfit.

"I've been asking her for years what her secret is. She won't tell me," Prince Harry said in a recent documentary – a sentiment echoed by Kate, Duchess of Cambridge: "I have no idea where she gets her energy from ... I need to go and, I think, ask a few more questions."

Such comments are played for laughs. In fact, the royal family has a ready, plausible and oft-repeated explanation for the Queen's Pope-like dedication to the job; her iron will to power.

She was always a serious girl, in contrast to her naughty younger sister. And she became a serious young woman.

Aged just 21, a few years before her father's unexpected early death, Elizabeth declared "my whole life, whether it be long or short, shall be devoted to your service and the service of our great imperial family".

As one relative described it, the shock of inheriting the crown at a young age in such sorrowful circumstances cemented the Queen's determination that her life was to be a submission to the role. Elizabeth was Queen, and could not be anything else.

"It appeared effortless but it jolly well isn't, of course," Prince Charles said.

Which is to make it a lot more dour-sounding than it actually is. Though the Queen is occasionally mocked for her grim expressions at public events, there are plenty of smiles to be caught as well – and not just when she's around horses.

Observed at one of the perennial Buckingham Palace garden parties, she stoically put up with God Save the Queen, a tune she must surely be sick to the back teeth of, but came alive for the "walkabout", shaking hands, asking "and what do you do"?

Part of her role now, and for a long time, has been to pass on such skills to her successors.

As Charles put it: "The horror of following along [behind the Queen] and thinking, 'how are you ever going to know what to say?' But if you follow along you learn, you pick up."

It's a deceptively difficult chore to get right, Kate told ITV: "There is a real art to walkabouts. Everyone teases me in the family that I spend far too long chatting."

In the documentary, William described the Queen as a figurehead providing stability, who "never dictated what we should do, it's her quiet guidance ... never seen but it's always there".

It's a nice line but there are signs it's spin on a more assertive substance. There is no doubt the Queen still decides and sets the tone and direction of the monarchy.

The Queen drums her sense of duty into the children and grandchildren. Contrast William's comment in the documentary, on home ground, with the way he put it in India: from the "safety" of the other side of the world William said the Queen was "a remarkably energetic and dedicated guiding force".

He signed off: "Now I can report back to granny that I have done my duty."

A force to be reckoned with, perhaps.

A new poll by Ipsos-MORI in the UK, published on Friday, found the monarchy was as popular as ever (76 per cent in favour compared with 17 per cent for a republic), and opposition to the Queen retiring has risen substantially since the turn of the century. Three quarters think the monarchy has an important role to play in the nation's future. Though 61 per cent think Elizabeth should be queen for life, about the same number think Charles would be a good king.

The situation is not quite the same in Australia - a place the Queen is likely never to visit again (though we seem happy with regular visits by Kate, William and George, and Harry). Last November a poll released by the Australian Republican Movement found 51 per cent would prefer an Australian head of state to "King Charles" when the time comes for him to replace his mother. Only a year before, support for a republic had been at a 20 year low, but it seems this may indicate loyalty to the queen rather than the crown.

Last year the Queen insisted that there be little fuss made of her passing Victoria's record – an "achievement" she didn't really see as such, more just an accident of her father's early death.

But this year she is more open to a celebration of her life.

On Wednesday the Queen will visit the Royal Mail office in the town of Windsor, a quaint, manicured place that embraces the castle with pubs, cobblestones and shops stuffed with tourist knick-knacks.

It's not just to check her face on the stamps. This year is the 500th anniversary of the postal service – subtly, she is pointing out that her lifetime is a modest fraction of the history of this civilised realm. Some staff are going to serenade her – a choir from the BBC reality series Sing While You Work.

Then she pops down to the local gardens to open a bandstand, where the theme is the 400th anniversary of the death of Shakespeare – another, almost certainly non-accidental signpost to the depth and strength of British history.

On her birthday the following day she will walk through the town, and that evening light a beacon which will set in train a series of more than 900 beacons across the country and worldwide. Beacon-lighting ceremonies will be the focal point for community parties across the country (royal parties are a fond tradition in Britain, excuses for a fete, stalls and fireworks, eagerly looked forward to by folk dancers).

Then there will be a private dinner for the royal family at Windsor Castle.

And then the work continues. The next day, US President Barack Obama arrives. And after that, the usual round of awarding honours, dealing with correspondence, receiving incoming and outgoing ambassadors, going through the government's red boxes every day and meeting the Prime Minister every week. It continues to be a full-time job.

According to palace insiders, the Queen wants to project calm and continuity for her birthday. She doesn't want the British to fear the end of an era is approaching.

One source, who did not want to be quoted directly, said the Queen had a sense it was a time of turmoil in the world. At such times her job was to be a symbol people could turn to, an emotional lodestone for the country and the Commonwealth.

In her latest Christmas message she quoted a Gospel of John verse about the "light [that] shines in the darkness, and the darkness has not overcome it".

Says the insider, the Queen's focus is still very much on the future. She does not want to dwell on her past, but to work to bolster projects that will outlive her – and the rest of us.

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