Research shows that there is an appropriate ratio of acute vs. chronic load to determine risk of injury; it is referred to as acute chronic workload ratio (ACWR).
The ratio is determined by the load (which can be thought of as mileage and intensity of running) over the course of one week compared to the average load over the course of four weeks. If the ratio is high, it means that the load over the course of one week exceeds that of the average load over the course of four weeks.
If the ratio is 1.0, that means that no change has occurred, and if the ratio is 2.0 that means that the load over the course of seven days is 2x the load that was accrued over the course of the month.
Research shows that someone is 2-4.5x more likely to experience an injury if the ratio is greater than 1.3.
The conclusion to draw from the recognized ratio is that injuries can occur when loads are placed in the body too quickly and without the appropriate progression.
It is important to challenge our bodies with increased run volume and intensity so that we can become stronger and faster runners. However, we will run into problems if we make dramatic changes to our training and do not allow time for our bodies to adapt.
Typically, it is safest to progress both run intensity and volume by 10-30% per week, so that the body has time to adapt to the stresses placed on it without significantly increasing the risk of injury.
Our bodies need to be loaded in a progressive manner, so that our body is better able to tolerate the stress that is placed on it.
Reference: Myers N, Mexicano G, Aguilar K. (2019). The Association Between Non-Contact Injuries and the Acute:Chronic Workload Ratio in Elite Level Athletes: A Critically Appraised Topic. Journal of Sports Rehabilitation. DOI: https://doi.org/10.1123/jsr.2018-0207.