Bill, the answer depends on each athletes training history. It’s helpful to understand the differences between TftNA and TftUA. The HR Zone guidance in TftNA is more relevant for elite and very highly-trained athletes, while the guidance in TftUA gives more consideration to a broader range of training backgrounds. The HR-based training guidance in TftUA is probably more appropriate for a majority of amateur athletes.
In this forum thread, Scott explains how he arrived with the TftNA HR-based Zones and associated training guidance:
For anyone else using TftNA, it’s worth understanding the difference. You can refer to the guidance in the training plan and numerous articles and forum discussions on the Zone 1 / Zone 2 allocation. Here are the comments from the Anaerobic Threshold Test:
{If the difference is 10% or less of the AnT HR then you will need to reduce the amount of Z2 training and substitute more Z1 for your aerobic base training or you will run the risk of overtraining by running too fast (Z2) too much of the time. If the spread between AeT and AnT is more than 10% then do all of your aerobic base training in Z2. Redo the AeT test that you did on day 1 each month if this is the case. As your aerobic base improves and your AeT HR and pace move upward you will need to adjust the volume of Z2 down.}
Here’s one example of what this looks like on the highly-trained end of the spectrum:
For highly-trained athletes who have spent years/decades optimizing their aerobic engine, substantial training in Zone 2 will be too fatiguing and unsustainable. To put this in perspective, AeT Pace for a world class marathoner (sub 2:10) is essentially race pace. The musculoskeletal demand is simply too high to do too many workouts at this intensity on a regular basis. For these types of athletes, Zone 2 training volume has to be carefully titrated. Zone 1 is more appropriate for most/many of their low intensity sessions. This is just one explanation why TftNA advises to train mostly in Z1.
For athletes who have not spent years or decades deliberately working on improving true aerobic fitness, much or all of their low intensity training can be done in Zone 2.
Each athlete is different, and the allocation between Zone 1 and Zone 2 will vary. The more training history you have, the more you will know what you can tolerate and what will be too much.