Mary A. Marshall, a Democratic delegate from Arlington, says being a member of the nation's oldest legislature, which opens here today, is a challenge -- "like the Matterhorn."

A surprising number of senior Northern Virginia legislators returning here for the 1980 General Assembly -- and some who chose not to come back -- say representing the interests of the Washington suburbs is a special burden not shared by their colleagues downstate.

The 27-member delegation toils in virtual anonymity at home, seldom holds "safe" seats and must wage costly reelection campaigns to gain seniority in contrast to many other Virginia legislators, they say.

The new session, expected to be dominated by intense haggling over tax cuts and tax increases in a budget year, is likely to prove a stiff test for the Northern Virginians, who find themselves unusually short on seniority and political clout.

"I'd just as soon be in Florida enjoying the sunshine," says a disgusted state Sen. Adelard L. Brault (D-Fairfax), who only a few months ago spent nearly $29,000 to hang onto his Senate seat and what he thought would be a new term as Senate majority leader.

Now, stripped of that leadership post by his own party last month, Brault has found that a lot of the fun has gone out of politics for him. More importantly, Northern Virginia has been left without any strong advocate for its interests in either house -- now controlled exclusively by downstate legislators.

"Northern Virginia has a reputation for not sending people back [to Richmond]," says Del. Dorothy McDiarmid (D-Fairfax). "I've had legislators from other areas of the state tell me they'd never run for office if they had to go through what we go through."

The fatigue of being a legislator is compounded for many Northern Virginians by the frustration of seeing some of their pet legislation fail year after year.

Proposals for funding the Metro system, for example, fell on deaf ears until the state's highway and transportation commission recently linked subway aid to increased highway construction revenues.

"I probably would not have run for reelection had I had any idea I would not be reelected majority leader," says Brault, 70. He had already been thinking of retiring when the region's most senior legislator, Omer L. Hirst (D-Fairfax) announced his own plans to leave.

"Hirst got the jump on me, quite frankly," says Brault.

Hirst was one of three Northern Virginians to quit the legislature out of a total of nine not returning this year.

The others who quit were former Alexandria delegates Richard R. G. Hobson and Gary R. Myers, a Democrat and Republican respectively. r

Hirst, 16-year veteran of the legislature and a wealthy landowner, wanted more time to spend with his family and to travel. Hobson found the needs of his law firm increasingly demanding. Myers found public service a tremendous strain on his business interests and family life.

And there was another discovery: "One thinks, wouldn't it be nice to be in the legislature?" Myers said. "But once you get there, you look around and think this isn't such a big deal after all."

More seasoned area legislators contend they get tremendous satisfaction from the feeling that their presence in the Assembly makes a difference for Northern Virginia. They say they've learned, however, not to expect the kind of praise and recognition at home that legislators receive elsewhere around the state.

"I have better name recognition in Richmond, Roanoke and Tidewater than I have in Fairfax," complains Brault. "People up in Northern Virginia are more oriented to the federal scene."

To their constituents anywhere else, assembly legislators are a prestigious lot, says Brault. "They're important people, we're not."

Del. Vincent F. Callahan Jr. (R-Fairfax), another veteran, says the longer a legislator serves, the more influence he or she has.

"You don't go into it for the money but you do feel you're accomplishing something," says Callahan, who, like most of the assembly members, is self-employed and better able to take time off from work for legislative duties.

He acknowledges that the job brings ego gratification, but says that kind of satisfaction "is offset by the harassment you get, the nasty letters and calls and the generally low esteem politicians are held in."

Northern Virginians find the trip between Richmond and home alone is time-consuming. Besides the 45- to 60-day sessions each year, more influential legislators find themselves in Richmond for meetings two to three times a week.

"Pretty soon, the only people who will be able to afford serving in our so-called part-time legislature will be millionaires and housewives," predicts Sen. Richard L. Saslaw (D-Fairfax), who successfully waged a tough race to succeed Hirst.