When you first start an exercise routine, you'll find that you improve rather quickly. Your mile time decreases and you start lifting more weights. Sooner or later this levels off and you reach a plateau. Your times and weights will start to stagnate. As you exercise, your body becomes used to the routine you throw at it. Only through change can this be corrected or avoided.
Over time, our body becomes more efficient at the exercise we do. More efficient means that it starts to burn less calories for the same activities. You get stronger and in shape so running a mile isn't as hard as it used to be. The only way to counteract this is to change your workout either with intensity, time or a new routine.
Intensity is how hard you workout. You up your intensity by making your workout harder. Run up a hill instead of on straight land (or increase the incline if you're using a treadmill or elliptical machine). You can also increase your intensity by running a faster mile.
If your pace is a 9 minute mile, shoot to improve it. This way you'll be able to run longer than a mile in 9 minutes and burn more calories. Note: running a faster mile alone may not increase your calorie expenditure.
Time is how long you workout for. You can negate your body's improved efficiency by running longer. Instead of running for 1 mile, gradually increase your endurance by adding half a mile or a mile every week. Add whatever you feel comfortable with. For some it'll be a mile and others quarter of a mile.
Every few weeks you need to change your routine. Your body gets used to what you throw at it and eventually your workout won't be effective if it doesn't change.
The type, order and intensity of your workout needs to change every month or so. Simply changing the orders of your workout is helpful as well as using different machines and weights.
The Bottom Line
If you find that you're stuck on a routine, then scrap it and make a new one. This will help you reach your goals and keep your routine fresh. An absence of change also makes your workout boring and if it comes to that, then you'll stop doing it all together.