James H. Duff has run the Brandywine River Museum almost forever, but at the end of this month his remarkable tenure as director, which began in early 1973, will end when he retires.

Except for four years at a small museum in the Hudson River Valley 50 miles north of New York City, he has devoted his entire professional life to Brandywine, most of that time wearing three administrative hats.

Guiding the museum's steady and impressive growth for nearly 39 years as director and chief curator has been only the half of it; in 1976, Duff was put in charge of the Brandywine Conservancy, the umbrella organization of which the museum is probably the most visible component.

The conservancy was founded in 1967, initially to prevent industrial development of the museum's site in Chadds Ford. It quickly grew to oversee protection of the 325-square-mile Brandywine River watershed, to prevent its despoliation by development, pollution, and other threats to its important environmental function.

The 68-year-old Duff announced a year ago that he would step down as of Dec. 31 in part because he believes the dual responsibility has become too expansive for one person to handle.

Consequently, on Jan. 1, Virginia A. Logan, a lawyer who lives in Rosemont, will take over as the conservancy's executive director. One of her first tasks will be to lead a search for Duff's replacement.

Considering how much the museum and the conservancy have grown since Duff arrived in Chadds Ford, it seems improbable that he was able to juggle both responsibilities for so long.

He could do so in part because he has worked in partnership with George A. "Frolic" Weymouth, cofounder of the conservancy and currently chairman of its board of trustees.

Logan's recent appointment confirms that the conservancy has become a full-time job in itself. "The job has gotten constantly larger, it needed to be bifurcated," Duff said.

Since its founding in 1967, the organization has negotiated conservation easements on more than 43,000 acres of watershed. It also owns and manages more than 2,600 acres in Pennsylvania and Delaware that include a number of historic structures.

One of the most visible and significant of these activities was the preservation of more than 5,500 acres of property in Chester County formerly owned by the famous King Ranch.

My involvement with Brandywine has always been on the museum side. There, as with preservation, growth has always been steady but unspectacular. Yet from the humblest of beginnings, the museum has come to occupy an important and recognizable niche in the region's art landscape.

It's hard to believe now, but, as Duff recounted, the museum germinated in June 1971 with modest ambitions - to be nothing more than a showcase for local artists, open only during the warm months of the year.

In hindsight, the fact that one of those locals was Andrew Wyeth, arguably America's most popular painter, should have suggested a more auspicious destiny.

From four dozen works, the permanent collection has ballooned to about 3,800, representing 400 artists. Annual attendance in recent years has averaged 100,000.

Duff says developing the collection (the title of chief curator also has been in his portfolio) has been his greatest source of pride. The most recent acquisition, not yet on view, is a painting by Horace Pippin, bringing the museum's total to four by him.

"What we're doing is working with selective aspects of American art, particularly landscape painting and the art of the Wyeths. We're not trying to become a general American art museum," he explained.

This was a sensible objective, not only because Brandywine is too small to attempt comprehensiveness - and why should it try to compete with Philadelphia's big museums? - but because the traditions of its location gives it plenty to work with.

First and most obvious, there's the Wyeth family, which made Chadds Ford a prominent art center. Many people think of Brandywine as the "Wyeth museum" because the various Wyeths, beginning with patriarch N.C. and son Andrew, are always prominently featured in the galleries.

Another leg of the stool is the region's connection with the Golden Age of American illustration through Howard Pyle, recently exhibited as part of the museum's 40th-anniversary celebration, and his students, including N.C. Wyeth.

The collection's third strength, perhaps less evident in public consciousness, is landscape, still life, and portrait painting of the Brandywine Valley, going back to the 19th century.

Just as the James A. Michener Art Museum specializes in Bucks County painters, the Brandywine has focused on famous locally connected landscapists such as Thomas Doughty, Jasper Cropsey, and William Trost Richards.

Trompe l'oeil (fool-the-eye) still life also enjoys a strong presence in works by locals such as George Cope and Jefferson David Chalfant. Consequently, the museum immerses visitors in a distinctive regional tradition, through both its collection and special exhibitions.

Duff also has guided the museum through two physical extensions of its original mill building, in 1984 and 2004, and through program expansion beyond its walls by acquiring and interpreting two significant Wyeth sites, N.C.'s house and studio and the nearby Kuerner farm, which inspired nearly 1,000 paintings and works on paper by Andrew.

Through an exhibition called "Three Generations of Wyeth Art," which traveled to five countries, beginning in the Soviet Union in 1987, Duff also projected the museum's presence internationally.

In retrospect, such a career seems improbable for someone who majored in English literature, both at Washington and Jefferson College, near his hometown of Pittsburgh, and the University of Massachusetts.

But Duff says he inherited an interest in art from his parents, and tipped toward working in an art museum when, in 1971, he participated in an international museum training program in Europe sponsored by the Metropolitan Museum of Art.

Brandywine proved to be the perfect situation for him, he said, because it combined art with natural history and environmental concerns, two other long-standing involvements.

In retirement, Duff won't be leaving behind the museum and the conservancy, or Chadds Ford; he's planning to write a history of the organization he has come to know so intimately, and to do other art-historical research.

That intimacy, and a succession of major challenges involving expansion of the building (twice) and programs, explains why he stayed in Chadds Ford so long.

"I've had 39 wonderful years here," he said, "it's just a wonderful place to be."

Contact contributing art critic Edward J. Sozanski at 215-854-5595 or esozanski@phillynews.com. Read his recent work at http://go.philly.com/edwardsozanski.