The world's longest and most exhausting bicycle race, the 3,120-mile Race Across America (RAAM), starts today in Huntington Beach, Calif., for 29 cyclists -- including three women and world-class professional Jonathan (Jock) Boyer.

After crossing mountains and deserts, averaging more than 300 miles a day -- often with less than 90 minutes sleep a day -- the riders will arrive in Atlantic City between July 30 and Aug. 2 and collapse on the boardwalk. The finishers will share $20,000 in prizes.

The duration of the fourth annual RAAM, which will pass through Virginia and Maryland for the first time, will depend on prevailing winds, storms, accidents and how much time the racers spend in the saddle -- that is, who can keep his or her eyes open longest.

It is a sleep-deprivation contest as well as one of the nation's most grueling sporting events -- likened by its promoters, the Ultra Marathon Cycling Association, to swimming the English Channel 21 times or running 50 Olympic marathons nonstop.

Boyer, from Pebble Beach, Calif., has been a top finisher in the Tour de France (12th out of 180 in 1983) and in world cycling championships and in 1980 won the Coors Bicycle Classic, America's 11-year-old Western mountain version of the Tour de France.

The men's cross-country record, 9 days and 13 hours, was set by last year's RAAM winner, Peter Penseyres of San Juan Capistrano, Calif., who is not competing this year. But his younger brother, Jim, who lost a leg in Vietnam, will compete with an artificial limb. He is one of a dozen riders who were barely under the limit in qualifying races and who are competing in the RAAM for personal reasons.

Among the three women contenders, experts predict it will be a contest between Shelby Hayden-Clifton of North Carolina, who holds numerous distance cycling records, and Susan Notorangelo-Haldeman of St. Louis.

The Race Across America is a difficult spectator sport. Riders may be dozens or hundreds of miles apart -- during a two-hour catnap a competitor can get 40-50 miles ahead -- and racers will pass through many communities in the middle of the night. RAAM cyclists prefer the cool hours from midnight to 6 a.m., when the nation's highways are largely deserted. It is not a popular time to watch a race.

However, if a fan knows the race route and the latest locations of riders from newspaper updates or by calling bike shops that are keeping track of the race, it is possible to estimate when riders will pass through. And local cyclists are permitted to ride along with the racers, although only behind the support vehicles, which must stay about 30 feet behind each rider.

In the Washington area, the nonprofit telephone bicycle service, HEY-BIKE (439-2453), will provide taped updates on RAAM and the location of the racers. The riders are expected in the Washington area July 29 or 30.

For nine to 12 days, the riders will do almost everything on their bikes, including doze -- falling asleep and off the bike is not uncommon. They also probably will eat chocolate chip cookies, since the major RAAM sponsor is the Great American Chocolate Chip Cookie Co., the nation's largest chain of cookie stores.

By the time the riders straggle up Virginia's Shenandoah Valley to Harper's Ferry and across the Potomac River at mile post 2,890, with only 230 miles to go, most will be near total exhaustion. Their speed will have slowed from an early average of 20 to 25 mph to less than 15 mph, if past RAAMs are a guide. That is about the speed of a Sunday cyclist on the Mount Vernon Bike Trail.