Washington will be the starting point for an estimated 1,000 long-distance bicyclists this spring and summer, as hundreds of bicycle convoys take 8-day and two-week loop tours of Northern Virginia and the C & O Canal 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, 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 still 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 4-to 7-week journeys to different points along the Trans-America Trail and 8-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. The route follows the totally car-less C & O Canal towpath to Harper's Ferry and from there either goes down the Shenandoah Valley or along Skyline Drive to Charlottesville. Bikers either would be transported by Bikecentennial truck back to Washington or pedal back through Fredericksburg and Mount Vernon.

In addition, Washington will have hundreds of day and weekend 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 teenage 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 Northern Virginia trips last weekend, two suburban Maryland trips this weekend - one in the Beltsville-Rocky Gorge area and the other around Poolesville - 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 Bicyle Atlas, which it prepared jointly withh the Washington Area Bicyclist Association, a nonprofit citizen action group for cyclists 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 Trial 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 trial, 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 Trial 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 Bikecentennail. 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.