Northern Virginia officially opens 26 new miles of the region's longest hiking and biking trail Saturday.

The 26 miles are part of the Washington and Old Dominion Trail, which is being hailed as a major acomplishment for the 22-year-old Northern Virginia Regional Park Authority. When completed, the 100-foot-wide trail will stretch 42 miles from the Potomac River to the Blue Ridge Mountains.

Local, state and federal officials are expected to join the public and members of the Park Authority in jogging, biking and riding a miniature train long part of the trail from Falls Church to Vienna -- where an all day Octoberfest will be held to celebrate the opening.

More than 600 area Boy Scouts are expected to take part in the celebration by hiking about 20 miles from Herdon to Purcellville. They will travel along western sections of the abandoned W&OD railroad where the trail is rugged and streams still must be forded or crossed by boat.

But the major activities for the public will be at the Vienna Community Center and City Hall, where visitors will find German food, beer, music and dancing and a children's circus. Those festivities will begin about 9:30 a.m. when area officials arrive in Vienna after a three-mile parade from Idylwood Park, at Virginia Avenue and I-66, just east of the Beltway.

Work on the $6-million trail, which is being built along the old W&OD railroad right-of-way, started almost two years ago when the Park Authority began the phased purchase of the right-of-way from Vepco.

If a pending $500,000 federal grant is approved, the trail will be paved next spring from Vienna to Loudoun County, and the trail's major bridge -- over Goose Creek -- will be built. And if Arlington voters next month approve a park bond issue to fund their county's share of Park Authority projects, the last section of the W&OD, in Arlington, will be bought from Vepco.

The new section being opened Saturday runs from Falls Church to the edge of Goose Creek in Loudoun County. Four bridges, which cost a total of $250,000, have been built over streams between Vienna and Goose Creek, making that section of the old railroad right-of-way usable by hikers and horsemen, and passable to hardy bicyclists in dry weather.

East of Vienna, the trail is now paved and includes a bicycle bridge over the Beltway and a bike lane on the Virginia Avenue Bridge over I-66. There is a short detour east of the Beltway, however, because the Virginia Department of Highways and Transportation tore up the W&OD right-of-way for I-66 construction and has not rebuilt it yet.

With paving of the Falls Church-Vienna link, it is now possible to bicycle from downtown Washington to Vienna almost exclusively on paved bike trails. However, the route follows meandering, narrow trails in Arlington and stops abruptly at two busy highways, Routes 7 and 123. The Park Authority has asked Falls Church and Vienna to install push-button traffic lights to eliminate the dangerous highway crossings.

The Park Authority so far has concentrated on buying and building the W&OD trail, and has not yet had the time or money to landscape it. While much of the Falls Church-to-Vienna section of the trail is lined with the flower and vegetable gardens of nearby residents, there also are concrete plants, car repair shops and dozens of industrial buildings and storage yards that abut or intrude on Park Authority property.

About 100 firms are using part of the W&OD under leases agreed to by Vepco. The park Authority must honor the leases for up to five years from the date it acquired each part of the old railroad right-of-way which for most of the firms will be until 1983. Park officials estimate rents from these leases, recently increased, are now bringing in about $35,000 a year.

In addition to the main trail, Arlington bicyclists ultimately will have two other bike paths along old W&OD spurs. One is along I-66, which is part of the I-66 highway project, running through Rosslyn to the Key Bridge, and the other, a Park Authority project, is along Four Mile Run Park to the Shirlington Shopping Center on I-395.

Arlington planners also hope to link the I-66 path with Roosevelt Island and the expanding network of National Park Service trails by building a bike bridge over the George Washington Memorial Parkway.

For Alexandrians, an extension is planned from the W&OD trail on the west side of I-395 to the George Washington Memorial Parkway bike path.