Human nature leads us to take it easy when we’re sore or tired. Obviously, we don’t feel good, so our nature tells us to rest until we feel better.
Many training plans for runners will ask you to run a high volume of miles, even though often times you are tired from the prior workouts. Many novice runners will make the mistake of skipping or curtailing the easier workouts because they are tired. They don’t realize their being tired is part of the training stimulus for those workouts!
In fitness training we have a concept called the Overload Principle. The principle is that your training stimulus has got to exceed your current capabilities to elicit optimal adaptions from that training.
For a beginning runner who doesn’t run much, the simple act of running in itself kicks in the overload principle. A beginner’s current capability is they aren’t yet comfortable running a lot. So running in itself already exceeds their current capabilities. A simple run will for them elicit those optimal adaptions.
Separately, consider strength training through weight lifting with heavy, challenging weights. Done with a suitable intensity (i.e. sufficient weight, capable but challenging form), lifting weights can exceed anyone’s current capabilities as long as the weight and/or exercise itself is more challenging that the trainee is generally used to. Even if a trainee gets comfortable with a given weight/exercise, adding weight or progressing the exercise into a more challenging form can once again exceed the trainee’s capabilities and elicit those optimal adaptions.
However, if the trainee were to maintain the current intensity as they got comfortable with it, the exercise while still beneficial would produce lesser adaptions and results. This is often why people hit a plateau when training.