All of today's programs including Hal Higdon's and Jeff Galloway's highlight the importance of the long run. Long distance aerobic running gives the strength and ground work on which much will be built. Lactate threshold training, speed work, and stamina will all come later, but the ability to run long has many benefits. VO2 max will increase from running within your aerobic training range. Capillaries will grow, enhancing the blood supply to the muscle fibers.
Training increases the number and size of mitochondria.
The mitochondria are the aerobic powerhouses of the cell. A variety of key aerobic enzymes will also increase. More myoglobin will be found in the skeletal muscle fibers. The significance of the increase in capillaries and myoglobin are the assistance that this will provide to the part of the VO2 equation specifying the difference in concentration of oxygen in arterial and venous blood, these changes facilitate oxygen transfer into the muscles. Strengthens the heart - larger stroke volume.