Replacing a hard run with an easy run can be one of the smartest decisions you make in training, but only when there is a good reason for it. The goal is not to avoid hard work. It is to make sure the hard work happens when your body is ready to benefit from it.
Hard sessions include things like intervals, tempo runs, hill sessions and demanding long runs. They are valuable because they create a strong training stimulus. But that stimulus only helps if you can recover from it.
If you start a hard session already carrying too much fatigue, the quality often drops. Your pace is slower, your form gets worse and the workout becomes a struggle rather than useful training.
There are a few situations where replacing the session with an easy run makes sense. One is when you have had several poor nights of sleep and feel unusually tired. Another is when your legs are still very heavy from the previous session. You may also need to adjust if your normal easy pace suddenly feels hard, your performance has been declining for several runs, or you are under a lot of stress outside training.
The important thing is to look for patterns rather than reacting to every small dip. Nobody feels perfect before every hard session. Feeling a little tired is normal. Feeling unusually flat for several days is different.
If you are unsure, start with the warm-up. A lot of runners feel stiff or tired in the first few minutes and then come right. If your body wakes up and your effort feels normal, continue with the planned session. If you still feel poor after warming up, there is no prize for forcing it.
An easy run keeps you moving and maintains your routine without adding the same level of stress. In some cases, the best choice may even be a full rest day, especially if you are sick, in pain or clearly not recovering.
What you should avoid is turning the easy replacement into a secret hard run. If you decide to back off, actually back off. Keep the pace comfortable and finish feeling like you could have done more.
You also do not need to make the hard session up immediately. Look at the rest of your week first. If there is a sensible place to move it and you are recovered, that can work. If not, let it go and continue with the plan.
The purpose of training is not to collect hard sessions. It is to improve. Sometimes the decision that helps you improve most is choosing an easier day.