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Train within your limits. Run and walk in intervals. Stick to an easy pace. Increase your mileage. Establish "normal" runs versus longer runs. Select a "medium" run. Keep your easy and medium runs fixed.
Whether you are only beginning to run or have already made a practice of it, design a training schedule based on your current abilities. Forget "no pain, no gain." Resist the urge to push yourself too hard too fast. Keep your runs sensible and achievable, especially in the beginning. Running is a high-impact exercise with a risk of injury, especially for beginners. The tortoise-wins-the-race mindset will decrease your chance of injuring yourself while gradually increasing your endurance. If you have little or no miles under your belt, alternate between running and walking in the beginning. Jog for one minute, then walk for four. Repeat three more times for a total workout of 20 minutes. As the one-minute jogging intervals become easier from one workout to the next, double them to two minutes each and decrease the walking intervals to three. Continue to increase the jogging and decrease the walking over time until you are comfortably running for a total 20-minute stretch. Keep it slow enough for you to be able to jog and talk with someone else at the same time without becoming seriously winded. Worry about developing speed later. For now, allow your body to adjust to the rigors of running without taxing it too much. Concentrate more on perfecting your form so it becomes more natural and less of a "pose" that you have to actively think about maintaining. Whether you plan your runs according to distances or set amounts of time, increase the amount you run in one session bit by bit. Be sure to make the changes gradually. For instance, once you are able to run 20 minutes at a consistent, easy pace, add another five minutes to your runs. Then, once 25 minutes becomes perfectly manageable, add another five to make it a half-hour run. Once you are consistently able to jog comfortably for a certain length of time or distance (let's say for 30 minutes straight), designate this length as your normal or "easy" run. In a given week, alternate between easy runs and longer ones. Again, increase your mileage gradually. On your first longer run, jog at your normal pace for 40 minutes, or even just 35. As this extra mileage becomes manageable, increase it by another five or ten minutes. However many days per week you are able to commit to running, always be sure to go out on more easy runs than longer ones each week. As you grow more accustomed to jogging for longer stretches than your easy run, pick a certain distance or amount of time as your medium run. Include at least one medium run in your weekly training, as well as one longer run. On your medium run, stick to the designated distance or time, while continuing to add mileage incrementally to your "long" run. Let's say you run five days a week. Your easy run is 30 minutes and your medium run is 40 minutes. On Day 1, jog for 30. On Day 2, jog for 40. On Day 3, jog for 30. Then, on Day 4, jog for 45 or 50 minutes. After that, finish your week with another easy 30-minute run. In general, the more mileage you run, the more damage your body incurs due to the constant impact with the ground. Once you establish your easy and medium runs, stick to those limits, whether you define them by distance or time. Focus on improving your performance within those limits rather than continue to endlessly expand them over time. Continue to add mileage a little at a time to your long runs if desired, but never do more than one long run per week. Give your body the chance to recuperate.