THE RODNEYS Murtaugh, ages 38 and 14, of Syracuse, N.Y., were standing beside their bicycles on a country road near the small Pennsylvania-Delaware border town of Hockessin, Del. This summer, picking unfamiliar sticky red berries (wineberries) from roadside bushes.

"We've never seen these before, but yesterday we picked blackberries without even having to get off our bikes," said the senior Murtaugh, who had taken his son on a motorcycle trip through the Great Smoky Mountains last year but decided that it went so fast they would take a slower and longer father-son odyssey and bicycle to St. Petersburg, Fla.

They had already gone more than 600 miles and had another 1,288 to go, most of it on the scenic but meandering back roads of the recently mapped East Coast Bicycle Trail.

Their small packet of trail maps, which fit into the plastic map case on top of a bike handlebar bag, provided "a new adventure each day with each map. We don't look ahead," said Rodney Sr. as they wobbled happily southward in mid-90s heat. Neither Murtaugh had ever done any cycling before, said young Rodney, "except for going around the block and things."

Hundreds of cycling tourists passed through Washington on the East Coast Bicycle Trail this summer and thousands of others followed the growing number of bike routes mapped out all across the nation, including the marathon 4,200-mile Trans-America Trail created in 1976 by Bikecentennial. The cross-country ride has become almost as much a rite of passage for cyclists who have the time as the 2,100-mile Appalachian Trail is for hikers. Each trail takes roughly three months to complete.

I did not have the time for a Virginia-to-Oregon passage, but I was planning to go to Maine for a summer vacation. As a teen-ager I bicycled around France and I have been riding two-wheelers fitfully ever since. So instead of driving 600 monotonous miles on I-95 and other interstates, I decided to take an extra week and ride 800 miles Down East up the East Coast Bicycle Trail.

There are basically two routes, a high road and a low road. The hilly high road takes you on back roads from Washington up near Frederick, Md., into Pennsylvania Dutch country and north to Poughkeepsie, N.Y., and through northwestern Connecticut. I took the flatter and hotter lowland route through Annapolis, Delaware, Valley Forge, Manhattan, Long Island and southern Connecticut, which includes two ferry boat rides, several high bridges and--unless you travel on Sunday--a hair-raising bicycle passage through midtown Manhattan.

The routes have much in common. They are largely on back roads with little traffic--in Manhattan, that means Fifth and Sixth avenues--that have been suggested by local bicycle clubs from New England to North Carolina.

It was on a hot July day near Worcester, Mass., after pedaling 80 miles and with another 40 or 50 to go, and feeling like Sisyphus as I climbed up hills with 30 pounds of largely unnecessary gear in my panniers, that I realized that the bike clubs had chosen the most interesting and challenging local roads to show travelers. After pumping up Purgatory Road in 95-degree heat, soon followed by a tortuous Christian Hill Road, I became a believer and left the official trail to follow a straight and narrow path, Rte. 109 into Boston.

The summer heat--it was 95 to 100 on six of the seven days I traveled--is never as oppressive on a bicycle because you create your own breeze. Your personal air conditioning system may evaporate two to three gallons of fluid a day from constantly refilled water bottles and fruit drinks and Gatorade from the general stores of every small town you pass.

Almost everyone recommends against drinking large quantities of soda or coffee. Long-distance bike racer Lon Haldeman said he drank "tons and tons of milk" in winning his second Race Across America in August (taking 10 days and riding just over 300 miles a day).

Another distance racer, Michael Shermer, set a Miami-to-Portland, Maine, bicycle record of 6 days, 2 hours in June, averaging 315 miles a day for 1,904 miles on major highways. He didn't say what he drank, but he did listen to music from "Chariots of Fire" and "Rocky" on earphones to take his mind off the hot, flat mid-Atlantic states.

The treeless back roads of the Delmarva peninsula were even hotter by the time I got to them in July, their tarry surfaces coming to a boil in the afternoon sun and bubbling like melted chocolate. The popping bubbles and the singing of the bike tires were often the only sounds, except for the buzzing of bugs and an occasional passing car or tractor.

It was 98 degrees when I reached Sudlersville, Md., "Home of Jimmy Foxx," on the afternoon of my first day out of Washington. The heat was relieved by a quart of orange juice and by accounts from the locals on the porch of the general store about how the home run king of the early 1930s could stand "right there (at the crossroads) and hit a ball all the way to the elementary school," where he used to go.

"Yessir," Foxx was called the "Beast," because of the muscles he developed swinging milk cans on his family dairy farm.

If you have a tent, as the Murtaughs and many touring cyclists do, you will find friendly farmers often willing to let you pitch it on the edge of their fields. I was tentless and looking that night for a motel. But the general opinion on the porch was that there weren't any until Delaware, some 30 to 40 miles away.

I climbed back in the saddle again, and as the town disappeared in the haze, I dropped down on the handlebars to stare at the passing potholes, gravel, broken glass and old soda and beer cans lining the road. Only in Maine and New Hampshire, the two states on my route with bottle laws, was the landscape unmarred by trash.

After two hours of daydreaming, I sat up to take a swig of warm grapefruit juice from a water bottle and saw in the distance across dry fields a mirage of shimmering spires: Middletown, Del., appeared like the Chartres cathedral rising above the plains of France. The spires turned out to be grain silos, water towers and a small church, and the town suffered from peeling paint, lack of attention and no motel.

I pushed on to "Historic Odessa" where, after repairing my first flat tire, I came upon a town of stately 18th-century houses with expansive lawns and views over the tidal marshes of the Delaware River. Nearby was a welcome 20th-century motel with a pool.

In the midsummer heat, it is the cool places that a cyclist remembers best: The early morning ride along Delaware River marshes, where rabbits sit beside empty roads and herons and egrets flap off lazily at your approach.

In the distance, an Intercoastal Canal bridge towers over the flatlands. You feel almost airborne flying down the other side into Delaware City at 40 miles an hour. Later, you enter the blissfully damp, cool stream valleys around Chadds Ford, Pa., where roads are dark, leafy tunnels and massive fieldstone farmhouses appear with almost every turn.

And at the end of a day, with the sun and a strong prevailing westerly wind at your back, you remember shifting into high gear and sailing across the sandy farmland of Long Island, arriving at Orient Point 10 minutes before the ferry leaves, or again with a tailwind, arriving in Boston just before dark--you have no bike lights--and finding a friend willing to take you in.

But the roads and hills and heat were not just things to escape. You feel a close, personal relationship with a hill you have struggled with, even if the feeling is not reciprocated. In Maine, along the state's historic coastal route, I even felt a oneness with U.S. Route 1.

Bicyclists take pride in having created pleasure tripping on American roads, with clubs like the League of American Wheelmen (LAW) paving the way for the Good Road Movement during the Gay Nineties--which some say the bicycle helped make gay. A kind of AAA of its day, the Wheelmen got state legislatures to macadamize the nation's first roads. It also published the country's first modern road maps and now publishes the East Coast Bicycle Trail maps.

Perhaps the major virtue of the maps is that they point out a continuous route through historic parts of America that are invisible from interstates and which passing tourists could not find on their own.

The roads I followed in the next two days in Pennsylvania and New Jersey were often in stream valleys--by old mills, farmhouses and former inns--and seemed to have changed little in 100 years. Children swam in ponds and streams everywhere, even in the Delaware River at New Hope and the Raritan River in New Jersey, where local residents proclaim the rivers clean enough again.

While the Wheelmen may be good on road maps, their hostelry information, however, is at best dated. Two youth hostels recommended on the back of the maps have been closed for more than a year.

On my third day, after a 130-mile ride into Staten Island across the high Goethals Bridge on a 24-inch-wide sidewalk, I followed the map along a decaying waterfront sweetened by the fragrance of a Procter & Gamble soap factory--with odors 99 and 44/100 percent pure. It led me to the Richmond Hotel, recommended by the trail guide, where I discovered the hotel had no air conditioning or fans to relieve the heat, and the manager advised from behind bulletproof glass that guests not go out at night because "it's too dangerous."

But renewal is coming amid the trash and ruins. Some neighborhoods are being restored, and above sidewalks where four-foot-high weeds grow in the cracks, billboards announced the coming of condominiums and a "Planned Recreational and Residential Community."

Touring cyclists seem to get a friendly greeting everywhere, especially from other cyclists. After my night at the Richmond, I got up at 5 a.m., because it was too hot to sleep, and cycled over to the ferry. There I was immediately hailed by a cyclist who turned out to be a bike messenger in the garment district out for a Sunday morning spin--a sort of busman's or bicyclist's holiday.

Ernest Prather, who is 29, has survived 14 of those years pedaling packages among the swirling trucks and taxis of lower Manhattan. There are lots of cyclists now in Manhattan, he said, and he frequently sees touring cyclists on Fifth and Sixth avenues. Prather then led me on a tour through the maze of Soho and Greenwich Village streets to show how easy bike travel is in the city. At least at 6 a.m. on a Sunday morning.

At 9 p.m. the next night in New London, I got off the ferry from Orient Point, Long Island, to discover the youth hostel was closed, so I turned to another League of American Wheelmen publication, a list of "Hospitality Homes."

"Sure. Stay with us. I'll be right down to get you in my pickup. It's too dark and difficult to ride over here now," said Ray Foss, one of several league members in the New London area who offer to put up passing LAW cyclists.

Foss and his wife, Mary Gail, both schoolteachers, live across the Thames River in Groton where they lead vigorous lives growing their own vegetables, sewing many of their clothes, heating their house with a wood stove--when they're not out on their tandem cycling around this country and Canada. (More than a dozen Washington area league members provide similar "Hospitality Homes" for touring fellow cyclists.)

And if the homes of friends are unavailable, and hostels, motels and New England's many new charming and inexpensive bed-and-breakfast homes are full, a stranded cyclist may even be invited in off the streets by local residents.

In York, Maine, a passing motorist with a car full of kids stopped to ask if I needed a place to sleep. I thanked him and rode on only to find out there were no rooms in the local inns. The York Chamber of Commerce, which has an emergency number for tourists, said in fact there were no rooms that night anywhere in southern Maine.

But as I sat on a graveyard wall, considering things and consuming a high-carbohydrate pizza and beer (not on the approved liquid list), a passing cyclist in black racing shorts stopped to see where I was from and offered a living room couch for the night in Kittery, 10 miles back--if I didn't mind following him first on a quick 15- to 20-mile evening training ride, as he was preparing for a double century (200-mile) ride.

On the following day, after struggling behind my benefactor on a quick 6 a.m. training ride back to York, I concluded en route to Portland, my ultimate destination, that the East Coast Bicycle Trail is a fascinating voyage to take once.

As for the cycling Murtaughs, "I don't think my son liked all the pedaling," said Rodney Sr., back home again in Syracuse.

"I think he liked the stops the best. But we had no major breakdowns and only one minor accident. I fell off my bike and the next day we bought helmets. And I think we'll look back on it, remember it, as an accomplishment. We made it."