Regular exercise, whether a long walk or a five-mile run, appears to reduce the risk of contracting colon cancer, according to a new study. Dr. Georgia Andrianopoulos, who conducted the study along with a colleague from the University of Illinois College of Medicine in Chicago, said the study of colon cancer in rats showed the risk was reduced by regular exercise. "The amount of exercise . . . is not as important as the fact that you do some," she said. Andrianopoulos, an assistant psychology professor, said the study did not examine why exercise reduces the risk. But she and Dr. Richard L. Nelson Jr., chief of gastrointestinal surgery, are continuing their research. The colon is the upper five or six feet of the large intestine.