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Push yourself. Avoid routine. Fuel up. Run smart.
Runner's high is your body's response to prolonged stress, so challenge it! Don't slack off or quit when you feel tired. Of course, "pushing yourself" may mean different things to different people, so interpret this according to your own level of fitness and experience. This could mean anything from:  Completing a 20-minute run. Running 90 minutes straight. Increasing your pace from a 10-minute mile to a 9:30 mile. Adding reps to interval training. Matching your last interval's time with your first's. As your running improves over time, expect your body to adapt to challenges already met. Since this lowers the chance of runner's high, keep switching things up from day to day. Even if you only have, say, 40 minutes to devote to running each day, find new ways to make each run different from the last. For example:  Alternate between easy runs and hard runs. Run different routes with different terrain. Switch between distance runs and high-intensity workouts. Challenge yourself by running faster paces. Keep your body energized with the right foods. Avoid tiring yourself out due to a lack of fuel before your body even has a chance to push itself for very long. Boost your energy levels with roughly 200 calories before working out. Depending on climate, drink between 8 and 16 ounces of water roughly 20 minutes after eating to hydrate yourself without impeding digestion. Give yourself at least 45 minutes to digest in between eating and running to avoid cramps. Keep in mind that getting runner's high is like catching lightning in a bottle. A lot of factors have to fall into place for it to happen, which are hard to predict or replicate, so don't make runner's high your main objective. Instead, focus on meeting performance goals. Increase your chance of feeling the high by constantly raising the bar for yourself.  Pushing yourself too hard can lead to early fatigue and injury, so set realistic goals for yourself. Wiping yourself out too soon or hurting yourself will prevent you from running hard enough to achieve runner's high. Runner's high occurs at different times for different people. Don't think you're doing something wrong just because a running partner experiences it and you don't when you run the same workout. What works one day may not work the next, or ever again for that matter. Keep your runs varied in nature, rather than doing the same thing day after day in an attempt to repeat a high.