Long before Jim Fixx was putting his bronzy calves on book covers and before George Sheehan was quoting Emerson, Goethe and Buddha on runner's high, Joe Henderson was pounding the roads and the typewriter. He was pre-running boom RUNNING A TO Z: An Encyclopedia for The Thoughtful Runner. By Joe Henderson. (The Stephen Greene Press. 192 pp. paperback, $8.95) and now he is post-running boom. The mass runs are thinning: Fewer people made it to Boston this April, and Washington's Home Town Run in May had a decline in numbers.

Henderson's first book on running, "Long Slow Distance," appeared in 1969, the Pleistocene age of the sport when an exorbitant entry fee for a race was 25 cents and when marathoners were seen as only slightly less freakish than the members of the Coney Island Polar Bear Club. Henderson, from Eugene, Ore., America's premier running town, has written eight books since then. That's a split time of nearly a book a year, which means Henderson is as fast with ideas as he is on his feet.

As a one-man support group for his national following, Henderson's newest offering is a winner because he is writing for all parts of the audience: those who have become run- down runners and need heart to keep going, the new converts with heads still free of running theories, and the yet-to-be runners for whom the most agonizing distance is the morning yardage between the bed and the shoes.

For Henderson, the A in his A to Z stands for "Approaches and Attitudes." He believes in pacing for a lifetime, not merely the next go-for- it-all 10K. "Running never has been, never will be, and maybe never even should be fun every step of the way," he writes. "Any writer who claims otherwise leads readers to expect something that running can't deliver. On the other hand, anyone who writes that running is always painful and boring misleads readers even more. He makes them think running is bitter medicine that people hate but tolerate because it's good for them."

Henderson is no misleader. He is a knowing analyst who understands that however many thousands of people have run in the New York marathon--the country's largest--there is the majority still crashed on sofa: "For every 100 people who think they might or should run, one does. Inertia keeps the other 99 bodies at rest."

Counsel for the one is kept simple. The first steps are inevitably uncomfortable. "After being hauled around (in cars) for most of the time, we have to learn to carry ourselves again. That takes a month for every year we have spent riding."

The critical point is reached in the post-euphoric stage when the good times have become slow times. This, Henderson says, is "when a runner must come to terms with the realities of running. No one passes through this critical stage unchanged. Some runners quit in depression or disgust, never to return. The wiser ones adjust their theories and practices, and come away from the crisis eager to move on."

Much of the finest writing in the past years has been on running. This is less a breakthrough than a continuation. Alan Sillitoe's "The Loneliness of the Long Distance Runner" remains a novel of strength, as does the autobiography of Roger Bannister and John Cheever's short story "O Youth and Beauty." Henderson's contribution to the literature is as a reporter doing his legwork: nothing flashy perhaps, nothing for talk shows. Running, he says "is quieting down and settling in to stay. The runners who remain will be the quiet and settled ones." Joe Henderson has been with them all along.