Whether you go for short walks around your neighborhood, sprint through 10-mile-bike rides, or sweat it out at the gym, at some point you'll think, "This isn't as hard as it used to be!"

That’s good progress. But it might be time to move on.

You may have gotten so used to your routine that it’s simply too easy now. To get faster or stronger, or just beat boredom, it's time to change things up.

It’s a fact: The best way to keep building muscle mass or burn calories is to make it a habit to switch up your workout -- especially when you notice that your current plan has become comfy. It keeps you on your toes. You’re ready to turn your walks into jogs, lift heavier weights, or swim faster than you might have thought you ever would.

Intensity: How hard do you exercise?

Do you reach your target heart rate? That will help you boost your fitness. Your maximum heart rate is about 220 minus your age. Your target heart rate zone is 50% to 85% of that.

If you don’t want to do the math, ask yourself if you feel like you're really pushing yourself, or if the activity feels pretty easy. Could you work a little harder, whether that means picking up the pace or adding more weight or resistance?

Time: How long are your workouts?

If you jog for 20 minutes, try to keep it going for 30. Strolling around the neighborhood for 45 minutes? Go a few blocks farther and make it 60.

Type: What specific exercises are you doing?

Consider going from walking to jogging, jogging to running, or add a completely new activity -- biking, swimming, Pilates, weight training, etc.

Consider a session or two, or more, with a certified personal trainer or exercise physiologist. They can tweak your plan, whether you're working toward a marathon or you want to lose those last 10 pounds. And they can make sure you don’t make too many changes, too fast.

If you're not interested in hiring someone -- maybe getting professional help seems too expensive or time-consuming -- you could team up with a workout buddy. It makes you more likely to stick with the plan and show up.

For the best results, pick a partner who's slightly fitter than you are. Research shows that working out with a person who's in better shape can motivate you to push yourself harder.