This article provides an overview of the almost-universal "zone" terminology used by many coaches and training-related articles. It's designed to give you a "feel" for each of these zones, not the associated power (Watts) or heart rate (HR) or "perceived rate of exertion" (RPE or PE) for each level of training.
There are numerous additional terms that are used to describe these basic concepts, and I won't even begin to attempt to write them all down. Suffice it to say, most everyone is conversant with the following terms even if they know or use a few more.
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Active recovery (Z1)
Easy, easy, easy. This is the region where you want to put in some quality conversational time with that significant other(s). This level provides little (but not no) training affect. Most coaches won't usually prescribe any specific amount of time to spend in this zone. They may schedule it after or before races to recover physically or prepare mentally. You can do all you want, but don't do it instead of a prescribed Z2/3/4/5 schedule.
Endurance (Z2)
This is the basic endurance-building zone, and should be at a conversational pace and still feel pretty easy. This is what's usually described as LSD ("long slow distance"). A significant amount of your training schedule (more than 50 percent) will be in this zone, even during your racing season, and it provides the physiological foundation for more intense training.
You should normally do at least an hour of Z2 training—and don't do a half-hour in the morning and the other half-hour in the evening. This does not provide the stress and adaptation we're looking for.
It is an excellent way to improve basic fitness, but will not help you get stronger or perform better. The benefits from Z2 don't really kick in until 45 minutes, and increase exponentially through 1 hour and 15 minutes, before starting to level off. Drink plenty of fluids before and during the workouts, thirsty or not.
With the foundation that you build here, the more intense efforts will provide significant benefits. Without it, the Z4/5 stuff will break you down swiftly.
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Typically, athletes will sacrifice Z2 workouts/time before Z4/5 whenever time runs short. If you do, you're guaranteed to burn out or overtrain. Don't worry; even the pros spend most of their time here. Of course, a pro cyclist (for example) goes 25 mph in Z2 where the rest of us do closer to 16 to 20 at the same HR; it's a function of genetics and training.
The direct benefits of Z2 training include fat-burning as the primary energy source, increased endurance, and stamina. Most people could do these types of workouts every day of the week.
Tempo (Z3)
This is where the work starts; usually referred to as Tempo rides or runs. You can still carry on a conversation, but the sentences get shorter and the level of conversation of a group decreases noticeably.
It's totally aerobic, and it should be a pace that you can handle for several hours. Use the entire zone, depending on the terrain. If you're scheduled for Z3 time, you'll probably have to work hard going downhill and not so hard going up those hills. This is the zone that you'll typically be at during races.
Get comfortable with recognizing this zone. If you're a cyclist, don't become a slave to your heart-rate monitor any more than your power meter (if you use one—they're a great training tool). You'll spend about 25 to 30 percent of your training time in this zone.