Don’t fear the red box

Gradual improvement comes from the accumulation of multiple sessions to change your homeostasis, no single session will have a profound change. You may do 300 or more training sessions a year. A 5-10% improvement for a well trained athlete over a season would be a good result. Thus each session only has a 0.0012% impact on your whole seasons improvement. Missing a few sessions isn’t the end of the world.

Forcing yourself to rigidly follow a plan which then results in you getting sick or injured, and thus missing weeks of training is not OK.