In order to achieve greater fitness, you workouts need to vary by intensity, duration, frequency and/or the mode of exercise.
Intensity: you can add intensity by changing the treadmill incline and/or the treadmill speed.
Duration: Increase the time you spend on the treadmill. This doesn't mean doubling your time two days after you first start your treadmill workouts. But if you have been doing 30 minutes on the treadmill for several weeks, bump it up to 45 and then to 60 for at least one session per week.
Frequency: Once you are used to treadmill walking, you can do it every day of the week. Walking at an easy pace for 30-60 minutes a day is recommended by every health authority as basic body maintenance. If you do tougher walking workouts on the treadmill and usually skip a day, add easy walks on the off days.
Type of Exercise: Try running on the treadmill to change it up. Or even better, alternate using the exercise bike, rowing machine or stairclimber. Don't use only walking for exercise. Add weight training, circuit training, anything you can enjoy and will get your body moving in new directions.