The following information is paraphrased from the Mayo Clinic’s web site…

When challenging yourself to create strength and flexibility, change how hard, long and often you work out. The trick is to avoid doing so much that you end up hurt or burned out. Make a smart and safe transition with these tips.

Start by assessing where you’re at now, as well as your strengths and weaknesses.

Consider:
What you already do (exercise mode), including cardio exercise and strength training
How hard you work (intensity)
How often you do it (frequency)
How long you do it (duration)
Set new goals

Next, take a look at specific, realistic goals you can set to improve your fitness level. Maybe you can jog or swim for 45 minutes, rather than 30. Or you could add flexibility exercises into your routine.

Do more
The best way to improve your fitness level is to increase your exercise intensity. Intensity refers to how hard you work. Pushing yourself outside of your comfort zone will help you to get the most effective results possible. If you exercise at a lower intensity, you’ll need to work out for longer sessions or more often to achieve the same fitness effects. In building up, first increase the frequency of your activity (number of days a week). As you become more fit, increase the length of each workout and finally the intensity.

To increase the intensity of your workout:

Move more briskly. The faster you move your body, the more work you’ll do within a given time.
Increase resistance.
For strength training, gradually lift more weight.

But don’t overdo it!
If you exercise too intensely, you run the risk of an overuse injury or fatigue and burnout. To avoid over-training, increase your total exercise time, distance or intensity gradually. Alternate hard and easy workouts from one day to the next, and build in time for rest and recovery.

Once you’ve reached a new fitness level, take a moment to congratulate yourself on how far you’ve come!