You can use the same approach and apply it to health and fitness. Challenges of any length keep you interested and help you prove to yourself that modifying your behavior is not all that hard.
- Eat only fresh foods for 10 days - no processed or packaged meals or snacks.
- Drink only water with every meal for two weeks.
- Eat a green vegetable with every dinner for one week.
- Exercise every day for 10 straight days.
- Abstain from alcoholic drinks for a month.
- Practice deep breathing exercises every night before bed for a week.
That's just a handful of examples. You can create a challenge of any kind and for any length with whatever rules you want to apply. Have fun with it and be creative.
You want this to be challenging but it doesn't need to be so difficult that it's discouraging. Anything you do is better than nothing and will improve your health.
Again, creating a your own challenge is an interesting way to create a new healthful habit, show yourself what's possible, and exercise your self discipline.