Camilla also marvels at the Queen's stamina as she continues to ride out most weekends.
"Who else is riding at 90? That's what's so incredible," says Camilla.
But even the Queen has her limits, it seems. The documentary shows Her Majesty being told by an aide that the weather forecast doesn't favour riding, prompting the Queen to admit: "It's quite difficult at this time of year... and I'm rather a fair-weather rider at the moment. I don't like getting cold and wet."
To make the programme, TV cameras were given access to the Queen's life for a year, from attending public events to simply relaxing at home.
She will celebrate her birthday on April 21 at Windsor Castle, but there are events around the UK until June to mark the occasion - the first time a reigning monarch has turned 90.
Camilla is joined by other members of the Royal family contributing to Our Queen At 90, which will be broadcast tonight on ITV 8pm Easter Sunday (UK time).
The Duchess of Cambridge tells during the two-hour programme how Prince George, two, affectionately calls his great-grandmother "Gan-Gan" and reveals that she once gave the Queen a jar of home-made chutney for Christmas.
World leaders congratulating the Queen in the documentary include US President Barack Obama, who describes her as "a source of strength and inspiration".
- Daily Mail