Washington will be the starting point for an estimated 1,000 long-distance bicyclists this spring and summer, as hundreds of bicycle convoys take eight-day and two-week tours of Northern Virginia or pedal in bike trains as far west as Colorado.

The trips from Washington are being sponsored by Bikecentennial, the nonprofit bicycle group which last year insugurated a 4,500-mile back-road-bicycle route across the U.S. from Williamsburg to Astoria, Oregon.

The Trans-America Trail, now approved and marked by the U.S. Department of Transportation and most state highway departments, will be followed again this year by hundreds of cyclists - last year more than 2,000, aged 7 to 73, crossed the country on it. The Eastern "trailhead" for the cross-country trips is stil Williamsburg. But far more cyclists are expected to take shorter one-to-seven week Bikecentennial trips originating here and on the West Coast.

The shorter Washington-based trips will include four to seven week journeys to different points along the Trans-America Trail and eight and 12-day trips on the Virginia Heritage Loop.

The two back-road that average fewer than 10 cars an hour, according to Bikecentennial planners who mapped it, and will pass by many Revolutionary War and Civil War sites. It follows the C & O [WORD ILLEGIBLE] towpath to Harper's Ferry then either goes down the Shenandoah Valley or along Skyline drive to Charlottsville. Bikers on the eight-day one-way-trip are then transported back to Washington by truck while those following a loop or circle tour then pedal a [WORD ILLEGIBLE] course past Ash Lawn, [WORD ILLEGIBLE] Fredericksburg and Mount Vernon, returning to Washington along the George Washington Memorial Parkway bike path.

In addition, Washington will have hundreds of day and weekend bicycling trips sponsored by local cycling clubs and also a few week-long trips. The American Youth Hostels chapter [WORDS ILLEGIBLE] many biking, hiking, canoeing and skiing trips, aimed primarily at teen-agers. It permits unaccompanied cyclists as young as 13 on its trips. The minimum age on Bikecentennial trips is 15.

The 1,100-member Potomac Pedalers Touring Club, which caters to older more experienced cyclists, has trips every weekend, including two in rural parts of Northern Virginia last weekend and in rural Montgomery County this weekend. AYH cycling trips do not begin until late March.

AYH will publish shortly a new edition of the Greater Washington Area Bicycle Atlas. Prepared jointly with the Washington Area Bicyclist Association, a citizen action group for bicyclists here, the atlas details more than 1,800 miles of scenic bike routes in the mid-Atlantic area. Included are a new back-road ride to Williamsburg, a tour of Antietiam Battlefield and a 100-mile circuit on Maryland's Eastern Shore. The atlas and information on Bikecentennial, AYH and other bike clubs and their tours are available at AYH area headquarters, 1520 16th St. NW, or by phoning 462-5780.

Bikecentennial staffers will be here Feb. 25, from their headquarters in Missoula, Mont., to put on a movie-slide show about the six Washington-based Missoula, Mont., trips they will sponsor from May to September and about the Trans-America Trail trips. It will be held at George Washington University at 7:30 p.m., building C, room 101.

Most cyclists participating in Bikecentennial trips - which are primarily camping trips that cost from roughly $100 to $900, meals included - will stay at the American Hostel here or a temporary "bike inn."

Four Washington-area residents were among the first dozen cyclists on the Trans-America Trail when it opened last May 16 in Williamsburg, but the group's most remarkable member was Clarence Pickard, an 85-year-old Iowa farmer who celebrated his 86th birthday pedaling into Kentucky. Pickard, who undertook the trip to demonstrate the need for self-reliance in America today, was forced to drop out in Farmington, Mo., after 1,500 miles, on doctors' orders.

While just over 2,000 cyclists completed the entire cross-country trip, another 2,500 pedaled along major sections of it, logging in all more than 11 million bicycle miles during the summer, according to Bikecentennial. Two of the cyclists were killed by cars, one while standing beside the road reading a map and the other when hit by a car traveling in the opposite direction. There were numerous skinned knees and a number of other injuries.

The Trans-America Trail will be virtually the same this summer, although about 100 miles longer where it has been rerouted to avoid gravel roads in the Midwest. Most cross-country trips will average about 90 days and can be made individually or as part of the 10 groups Bikecentennial is planning to lead.