When Jeff Hays was 15, he hiked the Appalachian Trail from Georgia to Maine. When he started, "I couldn't conceive of walking all the way." So he thought about the trek in less awesome terms, such as the next day's or next week's walk.

At 24, that day-at-a-time approach is what enabled him, he says, to complete a 375-page guide to 200 recreational sports in the Washington area. It is due off the presses this month after a year's work.

Hays, who also occasionally writes rock reviews, was working for a publisher of community telephone books when he came up with the idea. ("I've always been a little bit sporty, but I'm not a jock. I like doing things for fun.")

He had, however, no money to finance the guide. Making use of his advertising experience from the telephone book, he started selling ads to sporting-goods stores for the then-unwritten book. Somewhat to his surprise, the number of takers began to mount, especially after he completed a mock-up. As the deposits began coming in, he put together enough money to line up a typesetter and a printer.

Hays, who lives in Reston with his parents, admits that he teetered for many months on a financial tightrope. Now, however, he anticipates "making a ton of money."

He has sold about 400 ads, which should bring him between $40,000 and $50,000. Typesetting and printing 20,000 copies of the 7- by 10-inch paperback, he estimates will cost him $30,000, bringing in roughly $20,000 in profits from ads alone. Of the 20,000 initial copies (priced at $4.95), he will give 8,000 to advertisers to spur them into offering copies for sale. This step alone could solve the distribution puzzle. Providing the book is a hit, he expects to earn about $1 each from the remaining 12,000 copies.

But Hays' ambition doesn't end there. He is looking at the market for similar guides in cities across the country under the publishing name he picked as something of a joke: "The Brandex Corp." About half of the book--recommendations on equipment, addresses of national sports associations--could be used in any city directory. With a successful Washington guide in hand, he expects ad sales elsewhere to be easier.

Hays says he has had "a lot of fun" designing and laying out the book's pages. "I'm proud of the fact that I've figured it out. It's my little project."

But because the book has taken almost all of his waking time, he fears he may have lost many friends.

"I feel like I'm a bore."