Training should be fun and something that you look forward to doing! If there are things going on in your life that make training burdensome or feel unsafe, please listen to your gut. Here are some examples of things that may happen and how to deal with these situations.
If in doubt, leave it out - skip training on days when you feel like you are coming down with illness, are very fatigued, or have an evolving niggle.
If you miss a scheduled run, do not attempt a “make-up” session the next day - no make-ups!
In a 6 or 7-day/week plan, and sometimes in a 5-day/week plan, trying to make up a workout typically creates a cascade of unwanted schedule issues. No one workout is more important than the plan as a whole. If you miss a run, move on to the next and forget the one that was missed.
In plans that are 5 days/week or less, you might attempt to preserve runs by shifting them to runnable days, as long as the “no back-to-back higher intensity or LR days” guideline is not violated (also mentioned in the Modifying Your Plan section). If the run-shifting on one week would interfere with the following week’s schedule, don’t do it.
If the nature of the "life happens" that caused one run to be missed will have a carry-over effect ( fatigue/illness/injury) on the following run days, then consider making those days easy aerobic runs (or even take them off if necessary).
If one misses a run, some modest amount of volume sharing to the other runs in the week might be employed, adding a maybe 5-15 minutes easy running duration. Too much, and then injury risk goes up. Again, if in doubt, leave it out….just move on to the next run as scheduled.
If you miss more than three consecutive runs, then Stryd advises against jumping ahead to where you would have been had you not missed any days. Instead, Stryd usually recommends 3-7 days of easy aerobic running, then trying to re-enter the training the week just prior to when the problem caused the missed running. Of course, even longer layoffs typically mean restarting the plan from scratch.