Mr. Fixx's concern about his hereditary predisposition to heart disease - his father, Calvin, was first stricken at the age of 35 - contributed to his decision to take up jogging. When he began in 1967, to help rehabilitate a tendon pulled while playing tennis, he weighed 220 pounds and smoked two packs of cigarettes a day. He ran in his first race, five miles long, in Greenwich, Conn., in 1970, and Mr. Fixx finished last among 50 runners, impressed that the winner was a man in his 60's. As his passion for running increased, he stopped smoking, changed his eating habits, lost 61 pounds and proclaimed in the introduction to his first book on running that his purpose was ''first, to introduce you to the extraordinary world of running, and second, to change your life.''

Despite that bold claim, and Mr. Fixx's subsequent popularity on the lecture circuit as the guru of running, his book, which earned more than $1 million, presents a balanced account of the conflicting medical evidence about the connection between jogging and good health. He ultimately concluded that ''although the evidence is inconclusive, most of it clearly suggests that running is more likely to increase than decrease longevity'' because ''research has repeatedly shown that with such endurance training as running the heart becomes a distinctly more efficient instrument, capable of doing more while working less hard.'' Kept Elaborate Records

Mr. Fixx was known in running circles, and to his friends, as a man with a playful sense of humor about his avocation. He delighted in telling the story of the women he overheard at a cocktail party in his hometown of Riverside, Conn., who had made a mocking reference to ''that man who runs in his underwear.'' Besides keeping the elaborate mileage and time records common to serious runners, Mr. Fixx kept a log of the money he found on the road, which ranged from 21 cents in 1975 to $4.91 in 1979. ''Plus tools, like hammers, screwdrivers and wrenches,'' he said several years back, ''and even a wheelbarrow once. On Sundays, there will be cans of unopened beer, the residue of Saturday night parties, which I stash along the route and pick up later in my car.''

Mr. Fixx, born in New York on April 23, 1932, was graduated from Oberlin College in 1957 and worked as a magazine editor with publications including The Saturday Review, McCalls, Life and Horizon. He began his career as an author with two books called ''Games for the Superintelligent'' and ''More Games for the Superintelligent.'' ''The Complete Book of Running,'' published to coincide with the 1977 New York City Marathon, remained on the best-seller list for more than a year and was followed by ''Jim Fixx's Second Book of Running'' and an autobiography about his sudden fame entitled ''Jackpot.'' During his Vermont holiday, Mr. Fixx was to have put the finishing touches on his fifth book, ''The Complete Book of Sports Performance,'' which is scheduled for publication next spring.

Mr. Fixx was married twice - to Mary Durling and Alice Kasman - and was divorced twice. He is survived by his mother, Marlys Fuller Fixx of Sarasota, Fla.; his sister, of Santa Barbara, Calif.; and four children from his marriage to Miss Durling, Paul, John, Stephen and Elizabeth. Funeral services are scheduled for 3 P.M. Tuesday at Saint Savior's Church in Old Greenwich, Conn.