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 loop tours of the Virginia countryside 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 inaugurated a 4,500-mile, backroad 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 "traihead" for the cross-country trips is still Williamsburg. But far more cyclists are expected to take shorter one-to-seven week Bikecentennial trips originating here or 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, a rural route on roads that average fewer than 10 cars an hour, according to Bikecentennial. It follows the C & O Canal to Harper's Ferry then either goes down the Shenandoah Valley or along the Skyline Drive to Charlottesville. Bikers either would be transported by Bikecentennial truck back to Washington or pedal back through Fredericksburg.

In addition, Washington will have hundreds of day and weekend bicycling trips sponsored by local bicycling trips sponsored by local bicycling groups like Potomac Pedalers Touring Club and the area American Youth Hostels chapter. The AYH, whose trips begin in late March when below-freezing temperatures traditionally end, has many teen-age members and permits cyclists as young as 13 to ride unaccompanied on its trips. The minimum age on Bikecentennial trips is 15.

The 1,100-member PPTC, which caters to older more experienced bicyclists, has trips every weekend of the year, including two rural Montgomery County trips this weekend, and a special April "Bike Week" trip over Washington streets which the city is planning to mark as bikeways.

AYH will publish shortly a new edition of the Greater Washington Area Bicycle Atlas, which it prepared jointly with the Washington Area Bicyclist Association, a nonprofit citizen action group for bicyclists here. It details more than 1,800 miles of scenic bike routes, including a new backroad ride to Williamsburg, a tour of Antietam Battlefield and a 100-mile circuit ride along Maryland's Eastern Shore. The atlas and information about Bikecentennial, AYH and other bicycling groups and trips are available at AYH headquarters here, 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 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 Youth Hostel here or a temporary "bike inn." The trips are routed into and out of the city on the C & O Canal Towpath and George Washington Memorial Parkway bike trail, the Washington area's two longest and most popular bike paths.

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 first 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 Midweat. 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.

The Bikecentennial short and long trips from Washington will add to a greatly expanding number of like trips and bike trails in the Washington area.

The District, along with Montgomery County, is spending the most money on bicycle paths and programs - $600,000 over the next two years, with $400,000 of it federal funds. The city also has recently hired a full-time bicycle coordinator and has established a bicycle hot line to hear complaints and suggestions and give information about bicycle clubs or bike routes. The hot line, 629-4386, is operated weekdays 9-11 a.m. and manned by volunteers.

The city's bike coordinator, Eileen Kadesh (629-5280), who came to her job fresh from pedaling the 4,500-mile Bikecentennial route last September, is preparing a bicycle safety program for the District and helping department of transportation engineers map but those city streets to be earmarked as bikeways.

The District, which now has only one marked bike route, on Capitol Hill, plans to mark nine routes here as bikeways and to build two bikepaths, along Brentwood Parway and Watts Branch Creek, both in Northeast parks.