Four and a half years ago James F. Fixx, an obscure magazine editor and free-lance writer, became an instant celebrity. A book he had written with no particular expectation of commercial success, "The Complete Book of Running," hit the best-seller lists in the fall of 1977 and stayed there for a couple of years--ultimately becoming the most lucrative nonfiction title ever published by Random House, which is saying something, and making its author both wealthy and famous. In "Jackpot!" he tells his story.

It was, as such things so often are, a matter of being in the right place at the right time. The running craze that had swept through the upper-middle class was nearing its crest. Interest in the subject was intense, yet no authoritative manual existed to fill the appetite for information about it. Enter, quite by chance, James F. Fixx, who had received a modest advance for a book described in his proposal as a "general book on the subject . . . that tells you everything you need to know to get started, become fit, stay that way, and deal with the problems and pleasures you encounter."

Enter laughing--that is, all the way to the bank. It is a very rare occurrence for a writer to hit the jackpot, but Fixx hit it dead center at 100 miles an hour. His income for 1978 was nearly $600,000, more than $900,000 for 1979. Considering that since then he has also published an annual "Complete Runner's Day-by-Day Log and Calendar" and "Jim Fixx's Second Book of Running," it seems likely that from books alone he is, even after taxes, very much a millionaire. He is also very much a celebrity, recognizable not merely from pictures on his books but also from appearances on television talk shows and commercials.

Rich and famous: In the Age of the Media, that's the American Dream--but one that precious few of us will see fulfilled. So Fixx proposes to tell us what fulfillment is like:

"In a society that numbers among its journalistic ornaments such magazines as People and Us, not to mention uncountable gossip columns, no one can long remain in innocence about the manifestations of renown. It is, on the other hand, vouchsafed to few of us to witness the celebrity apparatus from within. This, once its tireless machinery had me in its soothing embrace, became my fascinating privilege."

He calls this book "a reconnaissance report based on the adventures I had" during a couple of wild years, when much of his time was spent promoting the book and reaping the fruits, some of them bizarre, of fame. It's a diverting tale, sufficiently stocked with amusing and/or appalling anecdotes to provide a glimpse into the gardens of celebrity. It is also an unwittingly self-revealing book in which Fixx emerges as faithful to a double standard: contemptuous of the tinsel and evanescence of fame yet thirsty for it, disdainful of the dollar yet willing to go the extra mile to earn it. It's a book that reflects considerably less credit on its author than he seems to imagine that it does.

He sees himself, or represents himself, as a babe in the woods: "I feel like a child who, digging on the beach for sheer pleasure, discovers gold." He describes his wide-eyed initiation into the rituals of television, autograph parties, endorsements, "being well known." Some of the stories he tells make valuable points: Tom Brokaw asking him a question on the "Today" show and then turning away while he answers, Susan Cheever Cowley conducting a "dictatorial" interview for Newsweek. The point--that there are beasts in the jungle--is well taken.

But the reader certainly must be forgiven for concluding that it did not take long for Fixx to become one of them. Freed by the success of his book from the cocoon of obscurity, he seems to have leaped at every straw, no matter how slender, that offered further wealth or fame. He describes, for example, several bonkers show-biz proposals he investigated; what he does not seem to understand is that the telling point is not that the proposals were bizarre, but that he was so eager to capitalize on them.

Ditto for the commercial endorsements. Again, he tells us about the products he declines to endorse, but the real point has to do with the ones he gobbled up: a travel card, a cereal, a soft drink. When the soft-drink folks came knocking, his agent turned down their offer of $15,000; but when they hiked it to $21,000, he jumped right on board.

Did Fixx need the money? Of course not. His book was making him wealthy; $21,000 was chicken feed in the game he was by then playing, and in any event he makes a major point of emphasizing how modestly he prefers to live. But he couldn't turn down the cash--or the recognition. Considerably more than he is willing to admit, he was and is the prisoner of his success, willing to scratch after every stray nickel.

"Jackpot!" is part of that process. Like the soft-drink commercial, it's a way of cashing in on success. The book is little more than a paste-up job, consisting largely of journals Fixx kept while writing and promoting the running book. It is his clear intent to make us admire him for his candor and modesty; but the sharp odor of opportunism makes that difficult.