So let’s say you have a running-related pain that’s not a stress fracture. How to proceed?
If you don’t have pain while running, but after, or when you get up in the morning, it’s OK to try running normal mileage at your normal pace. But keep good notes about the pain’s frequency and severity. And be honest with yourself about whether it’s not bothering you at all when you run.
If you can feel the ache when you’re running, but it doesn’t get worse during your run, you can try to carry on as per usual. When in doubt, stick close to home in case things get worse during the run and you need to shorten the run. (If it’s noticeable mostly at the start of your run, then it’s probably some tightness that you can get rid of with some flexibility work.)
If the pain gets worse as you run, something serious, or at least on the verge of being serious, is going on. Limit your run to less than the amount of time it takes for things to start deteriorating, and start crafting a rehab plan. You’ll probably want to add some cross-training so that you don’t feel guilty about running less and be tempted to keep running your normal mileage.
If the ache makes you alter your running form, you shouldn’t be running on it. Doing so will not only delay healing of what’s obviously a bad injury, but will also likely lead to injury elsewhere as your body compensates for your altered gait. In this case, it’s time to find a cross-training activity that doesn’t reproduce the running-related discomfort.
If the pain is present not only when you run but also during most of the day, you’ve let things go on for too long. You shouldn’t run and should cross-train only if you can honestly say the activity produces no symptoms.