Most of us are going to have to figure out what our Zone 2s are and what specific levels of effort called for in training programs mean for our own training. If you have the time, inclination, and budget that affords working with a personal coach, then so much the better. See Coaches
The references on this page are mostly for creating extended training programs to prepare for races. Most of the presentations include some details about how components of the plans work, like Zone 2 workouts. Here are links for more explanation of such workout components:
“No one ever stumbles over something while sitting down.”
Charles Kettering (paraphrased) (not referring to surfskiing, rather to the advantages of moving over being sedentary)
The training concepts described in the following discussions are applicable to all age groups. Younger athletes may be able to tolerate higher levels of exertion and will want to augment some of the recommended training levels to suit their own age and ability to recover more quickly.