Ten years ago, at the age of 35, Jim Fixx decided it was time to run.

Because running Army style was all he knew, Fixx slipped into a pair of heavy boots, went outside and started slowly shuffling down the street. Naturally he was unimpressed. Where were those wonderful benefits, the feeling of euphoria and general healthiness he had heard of?

Not long after, Fixx picked up a few pointers - primarily that wearing running shoes and lightweight clothing is far more enjoyable than Army fatigues and combat boots.

After one gourmet business lunch too many and a strenuous tennis games resulting in a minor leg injury, Fixx suddenly realized his days were numbered. "I knew that I was heading for a heart attack if I didn't change."

Angered by his body's betrayal and failure to resist the ravages of time, Fixx continued running sporadically. Now he averages 10 miles daily, has competed in over 20 marathons, and has written what some feel is the definitive book on the subject.

While there are many books out on nearly every aspect of running, "The Complete Book of Running" is the first to compile it into a virtual encyclopedia. An admitted reasearch freak, Fixx interviewed nearly everybody even remotely connected with running and read as much as he could on just about any facet imaginable. The result is a book to take you from the first jog right on down to training for the bigtime - the marathon.

Scientific data, running doctors, women, children, magazines, running clinics for heart patients, what to wear and eat - Fixx has covered it all. Not to mention how to improve time and distance, coping with foul weather, avoiding injuries, warding off dogs and slurs from onlookers.

Like most runners, Fixx is a "born-again." He knew that if he didn't cut out those lunches - where he "ate enough for two days at a meal," and smoking two packs of cigarettes per day - losing muscle tone and a youthful appearance would be the least of his worries.

"Running has become the most important aspect of my life," he says. "I stopped smoking without any effort. Running changes your values - then it's easy to change your whole life. I'm still basically the same person but I now live differently." Running is his antidote to life's ambiguities; accomplishing something physically difficult gives concrete results no one can dispute.

While some runners speak of the mystical aspects and meditational tranquility found in running, Fixx is more practical and sensible in his interpretation.

"It's an everyday vacation. I run long but not hard. There's no phone or things to be done. It's good for me physically and helps my mental attitude."

For beginners braving a brief turn around the block, Fixx offers basic advice:

Start slowly the first day and run less than you feel capable of. If you aren't sore the day after, try a little farther. Don't do too much too soon. Watch for body symptoms, such as extreme fatigue 15-20 minutes after a workout. Don't expect something for nothing. The "runners high" comes when you least expect it - after a month or two of regular training. Don't forget to invest in well-fitted running shoes.