The main theme of the book is that everyone could benefit from running slower. If you categorize running effort into high/medium/low, most people spend more time than they realize in the medium category. This middle effort creates fatigue in the body without the aerobic benefits of high intensity running. Therefore, the author makes the claim that approximately 80% of running time should be spent at easy effort and the other 20% should be spent at medium or high intensity. He supports this claim using studies in exercise science (although these studies tend to use experienced runners as subjects and small sample sizes) as well as anecdotes from elite runners throughout history.
The author also provides a lot of practical advice on implementing 80/20 running into your own training. For evaluating effort, you should be using a combination of heart rate zones, pacing, and perceived effort. He also makes a case for implementing more cross-training into your training plan. This effort is often lower-impact and can contribute to your overall aerobic time for the week. So if most of your cross-training time is low effort, then more of your running time can be devoted to higher efforts. Cross training is also an excellent way to maintain fitness if you are injured. The book also contains example training plans for several levels for races ranging from 5k to marathon.
I liked that there was a lot of discussion of research in the area. This approach is similar to other strategies like low heart rate training. This is appealing because I want to reduce impact on my body and become more efficient at all speeds. I don’t know if I will use these training plans specifically right now (figuring out heart rate zones sounds difficult at the moment), but I will definitely take the advice to embrace cross training while I work up my weekly mileage.