He started out as a park ranger at Bull Run Regional Park, when the Northern Virginia Regional Park Authority was a fledgling agency with fewer than 2,500 acres to its name. Over the next 31 years, as David C. Hobson ascended the ranks to the Park Authority's pinnacle, he helped it acquire and build such projects as Washington & Old Dominion Railroad Regional Park and expand the authority's holdings fourfold.

Now, Hobson says it's time to step down as executive director. At 56, he wants to do more writing and consulting work. With the Park Authority in solid financial shape, he's decided to retire at year's end.

"It's the most difficult decision I've ever made," Hobson said last week. "But this is a good time in my career for me to move on."

Replacing him won't be easy. "I hate to lose him," said Board Chairman Walter L. Mess. "We're advertising all over the country to find somebody of his quality. There isn't a job in there he can't do."

Mess credits the authority's first executive director, William Lightsey, with quickly recognizing the promise and potential in the recent college graduate who began working at Bull Run in the late 1960s. "He was so bright that we brought him into the office," Mess said of Hobson, "and put him under the executive director's wing."

Hobson became the agency's land acquisition director in 1972, then director of capital improvement programs in 1976. For much of the '70s, the agency's major project was the Washington & Old Dominion bike trail, a 45-mile stretch of abandoned rail line that winds through Northern Virginia from the Potomac River to the foot of the Blue Ridge.

During its creation, Hobson dealt with a host of problems, including building bridges, arranging street crossings, negotiating with landowners and pacifying utility companies. On completion in the mid-1980s, "it was one of the first highly successful projects of its type [the rails-to-trails concept] in the country," Hobson said. It now attracts more than 2 million riders annually.

The bike trail wasn't the only park in which Hobson played a key role. He also was involved in planning Meadowlark Gardens, a gorgeously landscaped swath of 95 rolling acres along Beulah Road in Vienna that opened in 1986.

"That's been a favorite project because of the different type of service it provides," Hobson said. Most of the authority's parks feature large, undeveloped areas, some with such tourist magnets as large outdoor swimming pools; Meadowlark, by contrast, has serene gardens, fountains, ponds and pathways.

Hobson expanded his versatility in other ways, too.

He learned how to write grant applications and ultimately helped the authority obtain more than $80 million in federal and state funds and land donations.

He became an expert in swimming pools as the authority began adding them to parks such as Bull Run and Algonkian and then began expanding to water parks with Cameron Run. When Cameron Run opened alongside the Beltway in Alexandria in 1983, it was the first park of its kind in the region.

The first authority pool, at Bull Run, also broke the mold. "Before that time," Hobson said, "swimming pools were L-shaped or rectangular. This was one of the first pools with an irregular shape. It was designed for fun."

Over the years, Hobson kept up with the latest in pool trends, modifying the authority's older pools "to keep pace with the expectations of the public."

Then there's the political hat, which Hobson wore even more often after becoming executive director for capital programs and administration in 1994. The authority is comprised of representatives from six areas--the city councils of Alexandria, Fairfax City and Falls Church and the county boards of Arlington, Fairfax and Loudoun.

Hobson said he spent "a substantial portion of my time dealing with intergovernmental affairs," trying to balance each area's interests.

Mess, who helped found the authority in 1959, said: "It's not an easy task, blending everybody together. But he has taken charge. He has political acumen. You have to have the ability to be the middleman."

Hobson became executive director three years ago, presiding over a period of slower growth than in the 1980s but overseeing a successful bond referendum last year that helped fund a $14.5 million capital improvement program.

He leaves some significant projects for his successor, of course, most notably the move to prevent development near Balls Bluff Park in Loudoun County. A landowner has proposed building homes on 20 acres of the Civil War battleground site; Hobson hopes the authority will be able to acquire the property through condemnation and stop the development.

Mess said the authority has appointed a search committee to find a successor to Hobson, adding, "We are sad to say goodbye to such a talented, effective public servant."

CAPTION: DAVID C. HOBSON