Taken from active.com:
The human body adapts very specifically to the demands placed upon it. If you run long and slow, your body will become very efficient at running long and slow. For example, if you run a total of 40 miles per week at a constant pace, you will train the energy systems and muscles of your body to do this very well.
However, if you never train at a faster pace, the energy systems necessary for that faster pace are left untrained. Studies have found that training aerobically (distance only) will not increase your anaerobic (sprinting) capabilities.
Our bodies are always learning and adapting to our training program. One of the reasons HIIT (high intensity interval training) is so effective is that it shocks our bodies. There are other programs that work better for other people. This is because your body has acclimated to your exercise program.
For best results, you should change your program every 4 months.