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As you improve, resist the urge to up your performance on your easy days. Better your odds at achieving runner's high on your next hard day (whether it’s a race or a workout) by making sure your body has recuperated from the last one. Remember to run at a pace that is slow enough for you to chat with a partner at the same time.  Tiring yourself out by running hard on your easy day will set you back on your next hard day. Starting a hard run when you’re already exhausted will reduce the chance of pushing yourself hard enough for your brain to release the necessary chemicals. At the same time, you're just as likely to experience runner's high on an easy day if your body still feels wiped out from yesterday's hard run. Once you successfully run 90 to 150 minutes a few times, start setting lower goals for your long-distance hard runs. Devote more time to high-intensity workouts, rather than accumulating more mileage per week. At the same time, reduce the chance of injury that may occur from so much repeated high-impact on your feet, legs, and hips.  Don't worry about losing endurance. High-intensity workouts will still help build endurance despite the lower mileage. At the same time, limiting the number of long-distance runs you go on will improve the odds of you experiencing runner's high the next time you go for one. Don’t expect runner's high to mask every single discomfort that you might feel. Take care of yourself so you can enjoy it more fully when it does occur. Always be sure to:  Warm up for 15 to 20 minutes by jogging lightly or walking fast before your run, race, or workout. Eat properly and stay hydrated, with enough time to digest food beforehand in order to avoid cramps. Wear appropriate clothing and shoes, both for running in general and the weather. Remember: there is no guaranteed way to achieve runner's high. Resist the urge to push yourself harder than your body can handle when chasing it. Avoid over-straining or injuring yourself, since these will only hinder your chances.
Keep your easy days easy. Cut back on mileage. Treat your body right. Don't hurt yourself seeking that high.