The combination of work and rest is effective.
On a scale of ten, most people's running intensity is around 5. Nowadays, top runners can't avoid this gray area. Their exercise is either intense exercise above 8 degrees or light exercise below 3 degrees. Why? The biggest effect of fitness comes from the most difficult exercise, but you can't reach the limit every time. By adopting the principle of "combining work with rest", you can get the maximum effect from the same amount of practice. On some days, you can exercise hard and then relax for a few days to let your body recover. This will help you avoid excessive exercise, and you won't get tired of the same exercise day after day.
For example, if you usually do level 5 exercise four days a week, you can try to run fast and long at 8 o'clock on two days and sprint and jog at 2 o'clock on the other two days. So your total training intensity in a week is about 20. It must be better than in the past to insist on combining work and rest for two months and then measure your own speed.