The reason people cried over Love Story is because I did," says Erich Segal about his 1970 best-seller. "I was innocent. I believed every word I wrote."

Now the more than nine million readers who shared Oliver Barrett IV's anguish when his Jenny died can pull out their hankies again. In Segal's just-published Oliver's Story, which picks up two years after Jenny's death, no one dies but, alas, Oliver's new love affair ends badly.

Not so with Segal's own life. In 1974 he met Karen James, a children's book editor, in London. For a month he bombarded Karen, who was separated from her husband, with letters and overseas phone calls. Then he returned to England and refused to leave until she accepted his proposal. Karen got a Reno divorce, and they were married in 1975. "I am genuinely and adolescently in love with my wife," says Segal, 39.

He credits Karen with straightening out his life. Love Story caused a "bad case of hyper-success that traumatized me," Segal acknowledges, adding, "I'd been a caterpillar. It was only marriage that got me out of my cocoon."

Karen, 30, a British-born diplomat's daughter, helped him finish Oliver's Story. "I couldn't find an ending that I myself believed in," Segal explains. She also did the final edit of the manuscript. "She's not kind with the blue pencil," he admits.

In the novel, Oliver meets blond divorcée Marcie Binnendale, a department store heiress, while jogging in New York's Central Park. Segal's characters are quick with four-letter words, but he is remarkably modest when it comes to sex scenes. On the final page, Oliver's romance has collapsed and he takes over the family business. Segal says, "That's it. There can be no third book."

Despite his popularity as an author, Segal is almost as well known as a long-distance runner. He began at 15 as therapy after his right leg was badly cut in a canoe accident. He has competed in 20 consecutive Boston marathons, never finished higher than 50th, but did break the three-hour barrier six times. Last month Segal finished a Faustian short story, Dr. Fastest, about a runner who sells his soul to the devil in return for winning the Boston marathon. Segal continues to jog up to 10 miles a day, donning a miner's lamp at dusk. "While the businessman is having his martini, I'm having my 10 miles," he says. "It's my only way of dealing with stress."

The son of a rabbi, Segal grew up in Brooklyn, got a Ph.D. from Harvard in 1964 and joined the faculty at Yale to teach classical civilization. He and Yale parted company in 1973 after Love Story turned him into a pop celebrity. "We were making one another unhappy." Since then he has been a visiting professor of classics at Munich, Princeton, Tel Aviv and Dartmouth. This fall he returns to Dartmouth as a permanent professor.

When he's writing at his home in Essex, Conn., Segal gets up at 3 a.m. ("I just can't sleep") and works with only "brownie breaks" until his late afternoon run. "I'm a window freak. I'm inspired by the light of dawn, and I love to write by natural light." Karen reports that he "yells and screams with delight when a passage comes right. He never gets moody or depressed."

His next novel, about "a lower echelon executive who in his late 30s discovers within himself a mysterious power," will be free of sentiment, Segal promises. He also has two academic works in progress: Problems in Plautus, a study of the ancient Roman playwright, and The Birth of Comedy, which traces the origin of that art form in Greece.

Oliver's Story has already landed on the best-seller lists and brought a reported $1.5 million for paperback rights. Segal is now winding up the screenplay and expects to be in production by late summer for release in 1978. Farrah Fawcett-Majors, is your agent listening?