What a few days off cost
Fitness fades slowly. A few days off, even a week, cost little aerobic fitness, and the rest often does some good. Two things do cost you: squeezing the missed sessions into the days after, and going back to hard sessions before your body is ready.
If you are ill
A common rule of thumb is the neck check. With symptoms above the neck only (a runny nose, a mild sore throat), an easy run may be fine if you feel up to it. With fever, chest symptoms, aching muscles or a stomach bug, don't run.
When you are back, keep the first runs easy whatever the plan says, and wait until you feel normal on easy runs before doing anything hard. After a fever, give it longer, and check with a doctor if you had chest symptoms or still feel unwell.
If you are travelling or swamped
A short easy run when you can is worth more than a perfect session you never start. If you can't run at all, let the days go. Don't try to make up the work on your return: pick the plan up where it is, not where it would have been.
When to rebuild the block
A missed week is a missed week. After about ten days or more off, though, picking the plan up where you left it means jumping straight into sessions sized for a runner who never stopped. At that point it is better to rebuild the rest of the block from where you are now, and to check the goal time again.
What Roen does
General training information, not medical advice. For pain that does not settle, or any symptom that worries you, see a doctor or a physiotherapist.