The travel agent thought I was joking when I asked for a one-way plane ticket to San Francisco.

"No, really, we're walking back," I said. Silence. "Well, maybe we'll ride bikes sometimes."

Of course, airline pricing being the way it is, we ended up with round-trip tickets anyway.

On Thursday, our scouting expedition started walking east from a Pacific Ocean beach. Eight months and 4,300 miles later we should dip our boots in the Atlantic Ocean, having mapped out the route for the first coast-to-coast hiking trail: the American Discovery Trail.

The American Hiking Society and Backpacker Magazine conceived this expedition because of a missing link in the national system of trails. Most of the major national trails -- like the Appalachian and the Pacific Crest trails -- run north-south following a major geographic feature.

The American Discovery Trail will go east-west through the nation's heartland. It will provide a way to discover America, passing through wilderness areas, small towns, and even some cities. It will follow historic routes such as the Pony Express Trail, traverse great mountain ranges like the Sierra Nevada and Rockies, wander across the plains and snake through the Appalachians and natural areas of the East.

On its way it will cross the north-south trails, providing a link among them.

That's the vision. But successful trails require another ingredient: People interested in making the vision reality. While this trail project is national in scope, each segment will be planned by local hikers and trail activists in the states through which the trail will pass -- California, Nevada, Utah, Colorado, Nebraska, Iowa, Illinois, Indiana, Ohio, West Virginia, Maryland, Washington, D.C., and Delaware.

The general route was chosen according to where public land is available, where scenic features are, and where people are available to hike it and work on it.

The local planners will put the route on existing trails where possible, then use their ingenuity to find hidden fingers of nature, greenways, abandoned rail lines, and bike paths through developed areas. Primarily a hiking trail, the American Discovery Trail will be shared with bicycles and horses in many places. In some areas, there is no choice but to follow back roads.

The local planners will guide our scouting team through their areas. And members of our little scouting team will go the whole way under our own power, by foot or bicycle, compiling data to write up a guidebook that will allow others to follow our footsteps.

But all this is only a beginning. The Appalachian Trail, opened for through-hikers more than 50 years ago, still has not settled into a permanent route. A trail becomes a living creature in and of itself, changing with time. Those who adopt the sections where they live will find ways to improve it, getting local landowners to give permission for the trail to cross their land and get it off a road, locating new parks and old, abandoned rail lines that become available in the future.

That's part of our mission -- to communicate to these people the possibilities the trail offers and get them to feel the connection of their segment to the national goal. Judging by the reactions of people we've contacted so far, the trail will have no shortage of supporters.

Before the "vision thing" becomes a reality, however, three scruffy people must walk across America, slogging along the trail, keeping notes for the guidebook, making contacts along the way, and doing our part to discover America. It's a dirty job, but someone's got to do it.

It's the first camping trip I've ever been on that started with a news conference -- in San Francisco's Cliff House overlooking the Pacific, followed by a photo opportunity at the Golden Gate Bridge -- and the first one with a commercial sponsor (Chevrolet and Coleman).

Other members of the scouting team are Bruce Franks, the leader, who left his position as assistant editor of Backpacker, and Ellen Dudley of Washington, the team lobbyist, publicist and logistics coordinator, who left a job as media director of the Union of Concerned Scientists.

I'll be reporting occasionally on our progress and adventures on this page. If you can't wait that long, the American Hiking Society has set up a hotline with updates on the progress of the American Discovery Trail Scouting Expedition. Call (703) 754-9008.

Eric Seaborg is a member of the American Discovery Trail Scouting Expedition and a former president of the American Hiking Society.