One of the biggest challenges athletes face isn’t the physical demands of training—it’s staying consistent. Whether you're an endurance athlete trying to improve your performance or someone just starting out, building sustainable fitness habits is key to long-term success.

But how do you turn a new routine into a lifelong habit? There are two popular frameworks for habit-building that can guide you: the 21/90 Rule and the 66-Day Rule. Let's dive into these principles and learn how to use them to create lasting fitness and health habits.

The 21/90 Rule: Building Momentum Early

The 21/90 Rule suggests that it takes 21 days to form a habit and 90 days to turn it into a long-term lifestyle. The logic behind this rule is that repeating a new behavior consistently for 21 days helps your brain internalize the action, making it a habit. Then, by extending that consistency to 90 days, you reinforce the habit until it becomes part of your identity.

Applying the 21/90 Rule to Fitness

For athletes, especially endurance athletes, this concept can be a game-changer. Here’s how you can implement it:

Start with Small, Achievable Goals: For the first 21 days, focus on simple, manageable fitness goals. For example, commit to running or cycling three times a week, or add two strength-training sessions to your schedule.

Consistency Over Intensity: Early on, it’s more important to focus on consistency rather than trying to do too much. The key is to show up for your workouts, even if they’re short or light. This builds the habit.

Tracking Progress: Keep a log of your workouts for the first 21 days. This could be as simple as ticking off a calendar or using a fitness app. Seeing your progress visually can motivate you to keep going.

Once you hit the 21-day mark, these actions start to feel more automatic. After 90 days, you’ll find it much easier to sustain the habit, whether it’s daily strength training, long runs, or prioritizing recovery sessions.

The 66-Day Rule: The Science Behind Habit Formation

While the 21/90 Rule provides a great framework to get started, research suggests that building true automaticity (the point where the action feels almost effortless) takes a bit longer. According to a 2009 study conducted by Dr. Phillippa Lally at University College London, it takes an average of 66 days for a new behavior to become a habit.

Dr. Lally’s research focused on how long it took people to adopt a new behavior and make it part of their routine. The results showed that 66 days is the median time required to form a habit. The study also found that some habits can take as little as 18 days or as long as 254 days, depending on their complexity.

Applying the 66-Day Rule to Healthy Habits

Strength Training for Endurance Athletes: For endurance athletes like runners or cyclists, incorporating strength training can sometimes feel like an afterthought. However, strength work is essential for improving performance and preventing injury. The first 66 days of adding strength training 2-3 times a week can feel like a grind, but sticking with it makes the routine feel automatic and turns it into a core part of your training.

Improving Nutrition: Nutrition habits take time to change. Rather than trying to overhaul your diet overnight, focus on making one small, sustainable change at a time. For example, you could start by adding more vegetables to each meal or switching out processed snacks for whole foods. After 66 days, these new eating patterns will feel more natural.

Rest and Recovery: Athletes often overlook the importance of proper rest and recovery, but without it, performance and consistency suffer. Building a habit of regular stretching, foam rolling, or scheduling active recovery days may take time, but after 66 days of consistent effort, you’ll start to feel the benefits of a well-rounded recovery routine.

Steps to Building Lasting Fitness Habits

Start Small, Build Gradually: Whether you’re trying to incorporate strength training, improve your nutrition, or develop a better recovery routine, start small. Focus on one habit at a time and build up slowly.

Set Clear, Realistic Goals: Instead of setting lofty, vague goals like "get stronger" or "eat healthier," set specific, measurable goals. For example, commit to strength training two days a week for the next three months or adding a post-run stretching session three times a week.

Use Habit Stacking: Habit stacking is a technique where you build new habits by linking them to existing ones. For example, if you’re already running three days a week, you could add 15 minutes of core strength work immediately after your run. Pairing the new habit with an existing one makes it easier to stick to. Alternatively you can pair habits with things you like to do. For example, you can watch a show/movie while you walk on the treadmill.

Accountability and Tracking: Track your workouts, whether it’s in a journal, an app, or a calendar. Regular tracking helps reinforce your habits and keeps you accountable. My athletes track their workouts as well as other things like nutrition, weight, progress photos etc., via an app.

Celebrate Small Wins: Habit formation is all about celebrating progress, no matter how small. Recognize when you’ve hit a milestone, like completing 21 days of consistent training, and use that momentum to push toward longer-term goals.

Be Kind to Yourself: It’s important to remember that habit-building isn’t about perfection. There will be days when life gets in the way, and that’s okay. What matters is getting back on track the next day.

Habits are for the long run, building healthy fitness habits isn’t a quick process, but the 21/90 Rule and the 66-Day Rule provide practical frameworks to guide your journey. By focusing on small, consistent changes, you can turn your fitness goals into daily habits that feel natural over time.

Whether you're an endurance athlete aiming to improve your performance or someone striving to adopt healthier routines, remember that building lasting habits is about consistency and patience. Start with achievable goals, stay consistent for the first 21 days, and aim for 66 days to solidify the changes. With time, these habits will become part of your lifestyle, fueling both your health and athletic performance for years to come.

Sources:

Clear, J. (2018). Atomic Habits: An Easy & Proven Way to Build Good Habits & Break Bad Ones. Avery Publishing.

Fiorella, R., & Mendes, L. (2013). The 21-Day Challenge: Success Habits to Achieve Fitness and Wellness Goals. Penguin Random House.

Lally, P., van Jaarsveld, C. H. M., Potts, H. W. W., & Wardle, J. (2010). How are habits formed: Modelling habit formation in the real world. European Journal of Social Psychology, 40(6), 998–1009. https://doi.org/10.1002/ejsp.674