You and your training program are like any good relationship -- variety will help it survive. You'll want to be sure to change things up. Heading out for a jog from your front door around your neighborhood is fine because it's the quickest path between you and an excuse-free run. But take the same course every day and you're likely to get bored. So head to the local track one day, find a trail through the woods or a park, or give yourself a weatherproof respite on the treadmill.
In addition to varying your training environment, you also need to be sure to change the angles at which you're pounding the pavement by including hills. Running up a slope will not only help you build strength in your legs, but also leads to a faster and more efficient stride [source: Men's Fitness].