(FORTUNE Magazine) – With the evening sun glinting off Lake Michigan, 6,500 runners from 400 companies raced out of Chicago's Grant Park in August on a 3.5-mile course that led through the Loop, along Lake Shore Drive, and back to the park. They were competing in the Manufacturers Hanover Corporate Challenge, a series of races sponsored by the New York City bank -- 14 this year, in cities from San Francisco to Killarney -- that will draw nearly 68,000 competitors. Winning teams travel to New York for a championship race along Manhattan's Park Avenue in November. For Nancy Moes, 22, who joined the Chicago marketing firm of Frank Lynn in June as an accounting clerk, the Corporate Challenge was not a good trip. A high school sprinter, she jogged recreationally at Valparaiso University. She was recruited for her firm's team just a week before the race and got in only four two-mile training runs before the race. Overheated as she headed into the last mile, she doused herself at a fire hydrant. On the last 200 yards, she threw up. She finished the race, but was taken to the medical aid tent. Her blood pressure and pulse were within normal ranges, but she was sick all night. ''Aerobic dancing I can handle,'' she says. ''I don't know about running.'' As legions of corporate runners pound purposefully across America, important new questions arise about exercise, running in particular: How much is enough and how much is too much? Many Americans are taking these questions seriously. According to a Gallup poll in June, 15% of adults over 18 now jog, down from a peak of 18% in 1984, when the Olympics inspired a surge of interest. Joggers are running less frequently, over shorter distances. The number of marathons is down from a 1981 peak of 353 to 310 races scheduled this year. Juggling a career and training for competition can cancel out the recreational benefits of exercise. Sarah Linsley, 28, an attorney with the Chicago law firm of Wilson & McIlvaine who competed in the Corporate Challenge, gets to work at 7:30 A.M. and trains evenings. Some days she finds she is just too tired. She has cut down from 60 miles a week to 45. ''When running is no longer a relaxation from the workplace, both are stressful,'' she notes. After ten years of racing -- she was good enough to be asked to the U.S. Olympic marathon trials -- Linsley thinks it may be time to switch to a less punishing sport like swimming or bicycling. Experts are reconsidering the amount of exercise needed for health. In 1978 the American College of Sports Medicine (ACSM), a professional organization of physicians, physiologists, and physical education instructors, recommended 15 to 60 minutes of continuous activity, three to five days a week. The ACSM said the activity should use large muscles in rhythmic endurance -- running, walking, swimming, skating, bicycling, rowing, cross- country skiing. The activity should raise the pulse rate to 60% to 90% of the heart's maximum rate to increase aerobic capacity, or the body's ability to use oxygen in physical activity. William L. Haskell, 48, director of the Center for Research and Disease Prevention at Stanford University School of Medicine and a past president of the ACSM, said in a recent speech that those standards may have been set higher than necessary. Haskell has concluded that exercise by sedentary individuals -- bicycling, jogging, or whatever -- at a $ pace slower than the ACSM recommended can significantly increase aerobic capacity. Dr. Kenneth H. Cooper, 54, head of the Aerobics Center in Dallas, a clinic and research organization, helped launch the all-American fitness boom when he popularized aerobic exercise over 15 years ago. Cooper has reduced his prescription. He suggests in a new book, Running Without Fear, that walking three miles in 45 minutes five times a week is sufficient for aerobic conditioning. Any more, he has said, and ''you are running for something other than fitness.'' Some women who exercise hard over a long time may face special risks. Scientists believe loss of body fat and hormonal imbalances may interfere with the reproductive cycle. Usually reversible when exercise is reduced, these disruptions were long considered benign. But last year Barbara L. Drinkwater, 58, a research scientist at the Pacific Medical Center in Seattle, reported lower bone density in 14 non-menstruating women, ages 18 to 35, who ran more than 20 miles a week. The mineral content of their vertebrae was at a level usually found in 51-year-old women with low levels of estrogen. Experts offer lots of arguments for exercise. Dr. Ralph S. Paffenbarger, 62, a professor at Stanford University's medical school, followed the histories of 16,936 men who entered Harvard from 1916 through 1950. He reported last year that the death rate from cardiovascular disease was almost twice as high among sedentary men as among those who burned up 2,000 or more calories a week in exercise. A study completed in 1981 by Dr. David Siscovick, 34, now at the University of North Carolina medical school in Chapel Hill, looked at the cases of 133 apparently healthy men in Seattle, with no known history of cardiovascular disease. All experienced cardiac arrest, nine while exercising. He calculated that for both sedentary and active victims the risk of cardiac arrest was elevated during vigorous exercise, but for habitual exercisers the overall risk was only 40% that of sedentary men. One runner who is far from throwing in the towel won the male chief executives' section of the Chicago Manufacturers Hanover race. Robert Barocci, 43, president of the international division of Leo Burnett, an ad agency, began running three years ago when his company's team drafted him for a half- mile leg in a relay. He next entered a 12-kilometer race, with miserable results. Realizing that competition requires a well-conditioned body, he got a running coach. On his lunch break he usually runs with the company club, and he logs more miles on weekends. Now training for the New York City Marathon October 27, he puts in 75 miles per week. Says Barocci: ''I feel healthier, and my wife says I look better, and that's important.'' What's most important, he acknowledges, is training carefully every step of the way.