Training Principles
The overload principle says that for the body to continue to improve and adapt, you must overload or provide more stimulus. For example, if you do 3 sets of bench press with 200 lbs for 8 reps every week, you will not suddenly be able to lift 300 lbs. You will simply get very good at lifting 200 lbs for 8 reps. You need to overload, and do more weight or reps each week, in order to improve your strength. This is where the SAID principle comes in.
The SAID principle is short for 'specific adaptation to imposed demand.' Basically what that means is that you will get better at what you do specifically. When you run long distances, your body adapts by improving your cardiovascular endurance. Running long distances will not help you squat heavy weights or jump higher. Conversely, building strength in your legs will not help improve your running unless you combine that with actual runs. Training in a way that is specific to you goals will give you the results you are looking for.