Each training zone does have its own unique benefits, but many of these training zones can also build on the some of same systems just in different ways. 
Our bodies don’t necessarily know about the different training zones and there isn’t an immediate internal switch when we move from one zone to the next. In some cases it’s a more gradual transition and as you shift from Zone 1 to 2, 2 to 3, etc.
This means that zone 2 and 3 while different, also have some similarities. One of the biggest differences as @Helvellyn mentioned is the amount of work you can sustainably do in each individual zone day after day, week after week. Athletes who train with really high volume (20-30 hours/week) often do a lot of lower-intensity (zone 1 & 2) work because they’re already doing a ton of high-intensity work. Since there are still many benefits to be had by riding at an easier pace, they tend to fill in each week with a lot of easy riding.
For athletes like you and me, it’s often going to be more beneficial to start with some of the higher-intensity work as it can be more effective in terms of boosting fitness, and as your training volume increases, you’ll want to start to add in more low-intensity work. Of course, there isn’t one singular way to train, and certain athletes do better with one approach over another for any number of reasons. 
All of our mid-volume plans and higher have at least two easy days scheduled. It’s just some of the low-volume plans that skip those easy days most weeks as we feel that you’d be better spending those few hours of training at a slightly higher intensity.
We have a few really good resources with info on this topic which I’ve listed below, and I’ve pulled out some of the key points and added them underneath for you to check out. 
Zone 2: Endurance
Also almost totally aerobic and relying on Type I muscle fibers, Zone 2 stimulates more adaptation and creates a bit more fatigue than riding at a recovery pace. Still, this intensity level is sustainable for a very long time, as its name indicates. Endurance rides increase the mitochondrial density, capillarization, and aerobic enzyme content of muscles, which all work to improve your aerobic capacity. The body is able to utilize fat for fuel at this intensity, and the ability to do so becomes more efficient over time. Endurance pace forms the bulk of most long rides, and during any mass-start event, you’ll spend lots of time here while sitting in the pack. In a training plan, Endurance rides are a great way to reinforce movement patterns and add training stress without adding much fatigue.
Zone 3: Tempo
Tempo is still primarily aerobic, but riding at this intensity begins to recruit some Type IIa muscle fibers. This is the first zone that feels challenging to sustain over long periods as it adds some muscular endurance demands. Tempo stimulates many of the same adaptations as endurance riding but with more pronounced effects, particularly in improving the abilities of the muscles to store glycogen. The fatigue created by riding for long periods at Tempo is much higher than at Endurance pace, but still relatively low.
Zone 4: Sweet Spot
Sweet Spot is a transitional zone, defined more by its applicability to training than by distinct physiological differences. It is the gray area between Tempo and Threshold and combines the most beneficial training aspects of both. This is how it gets its name, as the most useful and time-effective zone for general-purpose fitness improvements. Sweet Spot activates more Type IIa muscle fibers than lower intensities and triggers significant adaptations in muscular endurance, aerobic fitness, and glycogen storage. It is challenging and fatiguing to sustain for long periods, but not nearly as difficult or exhausting as riding at Threshold.
There are tons of other factors that play a big role in performance and cramping specifically such as nutrition and hydration, so I wouldn’t necessarily sell your fitness short!
Also doing body weight exercises a few days a week. Don’t have time to go to the gym.
