3. Don't increase your running mileage every week.
Run the same mileage for two to four weeks before increasing it. Give your legs a chance to fully absorb and adapt to the workload. You want 30 miles per week to be a normal experience for your body before increasing to 35 miles per week. And that takes time.
4. Don't increase the distance of your long run every week.
This is especially important if you're entering unchartered territory with your long runs (i.e., you've never run that distance before). Repeat the same long run for a few weeks before running longer. You want a 9-mile run to become normal before you try to run 10 miles. Most marathon and half-marathon training groups make the costly mistake of ramping up the long run too quickly because their training programs are only five to six months long, so they increase the distance of the long run every week throughout their programs until it's time to taper two to three weeks before the race. That's a good way for new or recreational runners to get injured because the stress increases week after week without a break. If you're running your first marathon or half-marathon and you're starting from a short(ish) long run, you need to give yourself much longer than five or six months to prepare without risk of injury.
5. Don't make the long run so long.
To avoid injury, don't make your long run such a large percentage of your weekly running. Ideally, your long run shouldn't be more than about a third of your weekly mileage. So, if your long run is 10 miles, you should run at least 30 miles per week. If your long run is 20 miles, you should run at least 60 miles per week. The majority of runners don't run that much, so you need to be creative when training so that you don't accumulate so much stress in one run.
Don't misunderstand--the long run should be stressful. After all, you're running for a long time and trying to make yourself exhausted so your body adapts. However, you don't want the long run to be so much more stressful than any other run during the week. It's always better to spread the stress around. Complete a medium-long run mid-week that's about 65 to 75 percent of the length or duration of your long run. This strategy helps to ameliorate the potential damage of your long run being more than a third of your weekly mileage.
Read More: 8 Baby Steps to the Marathon for Beginners
6. Run EASY on your easy days.
The biggest mistake runners make is running too fast on easy days. This adds unnecessary stress to your legs without any extra benefit and will make it more difficult to complete a quality run on your harder days. Easy runs should feel gentle and allow you to hold a conversation (about 70-75 percent max heart rate).
7. Never increase your weekly mileage and the intensity of your workouts at the same time.
When you begin to include interval training and speed work into your program, either reduce the overall mileage for the week or maintain your mileage from where it was before you added the extra intensity. Your legs can handle only so much stress at once. Trying to increase your running volume while also increasing the intensity of your workouts is too much for most runners to handle.