Running is a great sport. I have been, at various times in my life, very into running, or not into running at all. I love that it takes no special equipment, and that you can do it anywhere. It is also an amazing biological phenomenon, executing so many muscles and taking great coordination.
The major muscles involved in running include the quads, calves, and hamstrings, as well as the core muscles and hips.
Running is defined as having three phases, which repeat with each stride. The support phase is the time the foot is in contact with the earth. One foot touches the ground as the knee joint begins to flex. This is basically catching your body before you hit the ground. The body's center of gravity is typically above the point where the foot touches the ground. The primary muscles involved are the quadriceps.
The drive phase is the time the leg (that was preciously supporting the body) extends behind the runner. This pushes the runner forward and somewhat upwards. In this way, the leg continues to support the body from falling to the ground, but also propels the runner. The primary muscles involved are the quadriceps and various calf muscles.
During the recovery phase the foot loses touch with the ground and returns to the front of the body, where it will re-enter the support phase. In this way, each leg and foot is in a constant cycle between these phases while a person is running. The primary muscles involved are the hip flexors; this is the least intensive portion of the cycle.
Though running is typically considered a lower body sport, significant upper body movement is required, and competitive runners naturally develop strong upper bodies. This is especially notable in sprinters, as the faster you run, the harder your upper body works to keep balance. Your arms move in opposition to your legs, moving backwards as the opposite legs drive the body forward. This helps runners keep their balance and is a natural occurrence.
Running also release your body’s natural endorphins. With running (or any strenuous aerobic endurance exercise) your pituitary glands release these natural "feel good" hormones. While it may take a while for your body to adjust to the intense exercise that is running, I highly recommend it for a complete workout that will make your body and mind feel good!