The blush starts just above the collar of his navy Lacoste shirt and ends around his startlingly blue eyes. "The reports that I've had a lot of propositions," insists Jim Fixx, "are greatly exaggerated."

Rumors of Fixxated running groupies seem to embarrass the 48-year-old writer-runner whose 1977 best-seller, "The Complete Book of Running," displays on its cover a pair of glowing and well-muscled legs.

Fixx -- father of four who seems to be happily married to a woman who "runs three miles a day" -- said the stories started after he appeared on a TV talk show.

"One woman in the audience stood up and asked 'Whose legs are those on the book cover?'

"I said 'mine.'

"She said 'Will you marry me?'"

But for the most part, "having lived a life of relative obscurity" (as an editor for such magazines as Saturday Review, McCall's and Horizon), Fixx says "it's wonderful fun" to be known as one of the most intelligent spokespersons for the jogging generation.

Fixx had no dreams of fame 12 years ago when, as a heavy-smoking, "sedentary, 215-pound executive," he started running "in a spirit of self-preservation." This "positive addiction" helped him lose 50 pounds, "effortlessly" stop smoking, become calmer and gain "a sense of quiet power."

In addition to running 10 miles a day and editing, he wrote two "moderately successful" books on "Games for the Superintelligent." (Fixx is a member of the high-IQ club Mensa, but doesn't like to talk about it "because it sounds so snotty.")

Intrigued at the possibility of quitting his job and turning to full-time writing, he didn't have to look far for a book subject. "The Complete Book of Running" appeared, he notes, "just about the time of the big boom in running, which I had entirely unwittingly anticipated."

The success of the book and the sport which now has an estimated 40 million devotees, has made Fixx a minor star in commercials for credit cards he "doesn't leave home without" and in frequent TV and radio appearances.

So it's hardly a surprise that he's back, running down the promotional road again with "Jim Fixx's Second Book of Running."

"I didn't intend to write another book about running," he protests in a low-key manner that makes that believable. "But I have been in touch with doctors, psychologists and lots of people doing interesting things.

"The past three years have been the most explosive period in running, and I found myself getting an increasing amount of material. I told my editor it was time to revise the book, but after 'revising' the first six chapters it was about 95 percent new stuff. So I decided to just go ahead and write a new book."

But in calling his first volume "The Complete Book," Fixx acknowledges "I painted myself in a corner. The title in my contract was 'The Joy of Running,' but while I was writing someone else wrote a book by that name."

Then came "Come Run With Me," a title also usurped before Fixx finished. His editor picked "The Complete Book," claiming "one nice thing is that you'll never have to write another book about running."

Fixx valiantly unpaints himself in the Second Book's forward, pointing to the sport's skyrocketing popularity, the flood of new running gear and research on medical and psychological effects.

He includes chapters of the Age of Podiatry, the runner's mind, runner's food, running in childhood, past 40 and in foreign countries. But he's still leery about the "oversell" of running.

Oversell, such as "the doctor who claims if you can run a marathon in less than four hours you're immune from a heart attack for six years" has resulted, he says, in a "running backlahs" from non-runners and disappointed ex-runners.

"The wonderfulness of running has been oversold. A lot of times even the best runners have to push themselves through a run. If people start running and find it unpleasant, the best thing they can do is stick it out.

"Usually the benefits will become apparent. But if you go three, four months hating it, quit. There's no moral superiority in running. And it's important not to expect more than running can deliver.

"What you can expect is to lose weight if you don't eat a lot more, have more energy, a slower heart rate and generally have better health and feel better. Still I have some days I'd just as soon not run. But I've made a mental commitment. Generally I get out there and feel great."

Other runners feel great, too, judging from the "several cartons of letters" he's gotten from readers -- each one of which he answers personally "on my own rickety typewriter."

His favorite is from an Illinois steelworker who said he "weighed 340 pounds and was so ashamed of the way he looked he started jogging in place in his basement.

"He wrote me when he was down to 220 pounds, and I wrote back encouraging him and told him to write again when he got under 200. A few weeks later a package came from his the size of a shoebox.

"He said 'I'm now down to 180 pounds and to thank you I'm sending you something I no longer need.' It was a huge pair of pants with an immense waistline. That just about brought tears to my eyes."

Fixx's own running?

"It's never been worse," he groans. "There's nothing worse for running than writing a running book. All this insane traveling.

"I run every day, but I don't really train. Sleeping in different beds every night, not eating regular food. The other night I didn't like the looks of the hotel dining room, so I had three cans of beer and four candy bars for dinner. That's not typical, but it happens on the road."

His life style, however, hasn't changed much. His family still lives in their same house in Connecticut. ("We're not very materialistic.") He's "got a lot of ideas" and is writing "a non-fiction book that has nothing to do with running."

Will there be a "Jim Fixx's Third Book of Running"?

"It seems most unlikely now. I should have called this book 'Jim Fixx's Last Book of Running.' That would have fixed it."