Maria Puente

USA TODAY

You would think that upon reaching the age of 90 on Thursday, Queen Elizabeth II would sit back and put her feet up in her castle with a cuppa and a corgi. But not this nonagenarian sovereign.

The queen, accompanied by her husband Prince Philip, who turns 95 in June, will be working on her birthday, just as she has worked on most of her birthdays and on most days since becoming queen in February 1952.

Expect to see these two aged-but-still-fit royals on a walkabout outside Windsor Castle on the day, greeting well-wishers (many of whom will have camped out overnight to see her) after unveiling a plaque marking the Queen's Walkway, a four-mile walking trail in Windsor, the town surrounding the 930-year-old castle.

Not only that, Wednesday the duo carried out two other public appearances in Windsor of the sort that have filled her diary for more than six decades: She visited the Royal Mail office in Windsor to mark the 500th anniversary of the postal service, and met schoolchildren when she opened a new bandstand at the riverside Alexandra Gardens.

Prince George steals the show in royal family photo

Because that's the way she rolls — she's a woman devoted to duty, says CNN royal commentator Victoria Arbiter, who grew up around royals because her father is a former press secretary to the queen.

Arbiter says reaching 90 is a happy achievement for anyone but it's no excuse for slacking off in the view of the duty-conscious queen, who vowed service to her country at age 21 and has never retracted it.

"She’s doing engagements in Windsor on her birthday," Arbiter says, which shows what kind of model the queen is for her royal heirs. "She has provided the single greatest blueprint of any other monarch. She is a study of how it’s done, in every capacity — she is a study in how to be queen."

Later in the day, will be joined by members of her family, including grandson Prince William and his wife, Duchess Kate of Cambridge, for a private celebration at Windsor Castle in the evening.

And on Friday, she will host President Barack Obama for a private lunch at Windsor.

See more Queen Elizabeth birthday pics

It's a big deal that the queen is turning 90 — it marks another record for a record-setting royal. No other of the previous 40 British or English monarchs before her ever reached that age. And no other monarch before her reigned for a record 64 years; She passed that mark, set more than a century ago by her great-great grandmother Queen Victoria, in 2015.

She celebrated her Diamond Jubilee (60 years on the throne) in 2012, and looks to be headed for her Platinum Jubilee in 2022; after all, her mother, Queen Elizabeth the Queen Mother, lived to be just short of 102.

But the big whoopee for the queen's 90th will be celebrated by the British mostly in May and June, when the weather is better and the queen's birthday is normally marked at the Trooping the Color, the annual parade of horses and troops in scarlet uniforms and tall bearskin caps in central London.

For three days starting May 12, Windsor Castle will be the setting for a 90-minute extravaganza with 900 horses and more than 1,500 riders, actors, bands and dancers from around the world, who will tell her story from her birth through to World War II, her marriage, her coronation and her reign. The queen will attend on the final evening.

On June 10, her birthday will be marked with a televised Service of Thanksgiving at St. Paul's Cathedral in London. (June 10 is also Prince Philip's birthday but he doesn't want any public celebration.)

The following day will be the Trooping the Color ceremonies and the day after that, June 12, will be the Patron's Lunch, to celebrate the queen's patronage of more than 600 charities and organizations.

This event, organized by the queen's eldest grandson, Peter Phillips (son of her only daughter Princess Anne, the Princess Royal) will be unprecedented. The Mall in London is to be transformed into a giant street party and picnic for 10,000 paying guests, with entertainment by street performers and circus acts, while the queen and her husband watch from a specially-built platform on the Queen Victoria Memorial in front of Buckingham Palace.