Jim Fixx popularized running and helped to pioneer the fitness revolution
Jim's books were and still are popular. The Complete Book of Running is still a standard for marathon runners. He's much of the reason why running has become the most popular form of exercise in the Western world.
He was a fitness pioneer much like Jack Lalanne. Both saw exercise as a crucial part of overall longevity and health. The point they differed on was that Jim believed exercise was the key and diet and nutrition played a much smaller role.