Smoking negatively impacts physical fitness and endurance by restricting lung function, reducing oxygen transport, and increasing heart rate. The toxic substances in cigarette smoke impair the body's ability to utilize oxygen efficiently, which is vital for aerobic activities. Additionally, smoking can lower overall cardiovascular health and diminish stamina, making it difficult for individuals to perform at their best during workouts. Quitting smoking can lead to significant improvements in physical fitness levels, allowing for enhanced performance and recovery during exercise.