But if you can't run yourself into the ground in the first year of your dream job, then when can you? The London-born Penny says returning to the National Gallery is like coming home, having worked there for 10 years as a curator in the 1990s. He left in 2000 for a job in the US, but came back to Trafalgar Square in February after Charles Saumarez Smith resigned to join the Royal Academy.
Since then he has been working hard to reacquaint himself with the gallery and its staff. "I think it is best to do things yourself before you delegate them, so you know what you are delegating, so if I look tired…"
Despite his fatigue, there is no let up. The National Gallery's and National Galleries of Scotland's joint purchase of two Titian painting owned by the Duke of Sutherland is the latest addition to his workload.
They are part of the Bridgewater Collection, which is currently on display at the National Gallery of Scotland and also includes works by Rembrandt, Raphael and Poussin. If £50m can be raised by the end of the year, they will be able to purchase one of the Titians and will be offered the other one, again for £50m, in four years' time. Buying both paintings will allow the remainder of the Bridgewater collection to remain on long-term loan in Edinburgh.
Penny has been lobbying hard to persuade the public, private benefactors and government that the Titians are worth saving. "It's rather hard to think about anything else," he says.
But despite seeming happy to work himself into the ground, he is worried about the strain on other members of the National Gallery's staff. His main concern is the stress put on curators by the gallery's relentless pursuit of blockbuster exhibitions such as those on Raphael, Titian, Velásquez and Caravaggio that were held under the stewardship of his predecessor.
"The strains of putting on great exhibitions of that kind have begun to tell on the curatorial staff," says Penny. "It is more and more difficult for them to think up new things of a popular kind to do. And it is more difficult for them to get on with some of the other things that curators should be doing."
Penny says he is not against temporary exhibitions, it's just that he wants a better balance between the time and resources devoted to them and the permanent collection.
"We certainly want to carry on putting on exhibitions, but if we aim to put on hugely popular ones, we can only do so at the expense of our permanent collection," he says. And it is the permanent collection that Penny believes is the heartbeart of the National Gallery - for staff and visitors.
"The chief point of the permanent collection is that it is there to be revisited," he says. "A great novel you read and read again, a poet you read and re-read. Many of these works of literature are only designed to be understood when they are re-read, and so it is with great paintings; they are things you revisit. This is not possible in a temporary exhibition."
Encouraging public curiosity
The trick for Penny will be to translate his passion for the permanent collection to the public. He says that part of the National Gallery's role is to revive the public's interest in Old Masters, but you get the feeling that he could be frustrated by the levels of people's interest.
"Above all, I'd love to see more of a culture of public curiosity where people want to go to an exhibition about an artist they have never heard of. I'd like people to take much more interest in going to see things which they haven't been told tremendous amounts about. [At the moment] you get into this culture where people's priorities are being driven by the museum's own publicity.
"If you highlight 10 masterpieces in a museum, you are saying most people won't want to see most things and you are betraying the whole idea of self-education, which is fundamental to museums with public collections, where people go and do their own thinking."
A serious art historian
Many people use the word "connoisseur" when talking about Penny, and Rusty Powell, his former boss at the National Gallery of Art in Washington, describes him is a "gifted and serious art historian" with a "deep intellect".
This reputation has come from his extensive writings and in-depth studies of collections. High-profile evidence of this knowledge came with his discovery of Raphael's Madonna of the Pinks at the home of the Duke of Northumberland in 1991. The painting was later bought by the National Gallery for £22m.
But despite Penny's scholarly reputation, he will have to get his hands dirty dealing with some more down-to-earth problems at the gallery, such as the pay dispute with the 50-plus members of the Prospect union, who went on strike in July.
And he will have to keep a close eye on attendance numbers, as the gallery attracted 2,380,000 visitors from January to July this year, about 285,000 fewer than the same seven-month period last year.
Penny's passion for the scholarly side of collections is no surprise, as he started his career teaching art history. His great mentor and friend was the Oxford University art historian Francis Haskell, who died in 2000.
"I had no plans whatsoever to go into the museum world," Penny says. "But I had a very strong supporter and close friend in Francis and I think he understood that many of my interests would really suit being a scholarly curator. When the job of appointing a new head of the department of western art at the Ashmolean came up, I was encouraged to apply by Francis. He was right. I immediately loved it."
After the Ashmolean he joined the National Gallery as the Clore curator of Renaissance painting. Then came the eight-year period in the US. He applied for the top job at Trafalgar Square in 2002, but it was given to Saumarez Smith. He was more successful the second time.
"It's good to come back. I have lots of friends here," he says. "I was born in London, so it's coming home. And I think it is a great advantage for me to have been away from the gallery as I am both an outsider and an insider."
So how will Penny's approach to the National Gallery differ from Saumarez Smith's? He announced his 2009 programme last month and it will include a Picasso show, which will look at how the artist was inspired by Old Masters.
There will also be an exhibition on impressionism and another on the influence of sculpture on 17th-century Spanish art. In all of these Penny has tried to place a greater emphasis on links to the permanent collection.
Penny has also identified various gaps in the collection, such as German and Italian 19th-century paintings, so more acquisitions can be expected in these areas. He would also like to improve the museum's website, and further down the line, expanding the National Gallery site is a possibility, but he wants to be clear about "what exactly we are expanding for".
He has a long "to do" list, but perhaps when some of them have been achieved, he will feel able to put up his feet and admire an Old Master or two.
Nicholas Penny was the Clore curator of Renaissance painting at the National Gallery between 1990 and 2000. He then went to the National Gallery of Art in Washington as the Andrew W Mellon professor at the Center for Advanced Study in the Visual Arts and in 2002 became the senior curator of sculpture and decorative arts.
He returned to Trafalgar Square as director in February 2008 after eight years in the US.
His first museum position was as the keeper of the department of western art at Oxford's Ashmolean Museum.Before this he was a lecturer in art history at the University of Manchester before moving to Oxford. He has a doctorate from the Courtauld Institute of Art.
Penny was born in 1949 in London.