As part of his daily exercise, Calvin takes a walk a fixed course. He generally does so at the same speed. On a day that he's feeling energetic, he walks \(2\) km/hr faster than normal, and found that it took half the usual time plus \(15\) minutes to finish the course. On a day that he's feeling lethargic, he walks \(1\) km/hr slower than normal, and found that it takes \(30\) minutes more than the normal time. What is the distance (in km) of the course that he walks everyday?