“Happy hard”! I hadn’t heard that before, but it’s perfect.
First, definitions: The “junk zone”, the “black hole”, etc in a five-zone system is Zone 3. The zone label may vary, so the thing to keep in mind is that it’s an intensity that is bracketed by the aerobic and anaerobic thresholds.
In athletes, it’s an overstatement to say that it should be avoided. But that’s good advice for the general population. It needs to be trained like every other intensity. However, most people go there too often because of the happy-hard attraction. Like @briguy said, it’s hard enough to feel like work, not hard enough to get a lot of benefits, and not easy enough to build aerobic capacity. When it’s a high proportion of training time, it’ll reduce aerobic capacity.
As the aerobic system strengthens, Zone 3 narrows. It can get so narrow (as @davelockyer101 has described) that it functionally doesn’t exist.
BUT! When that happens, Zone 2 training becomes just as exhausting as Zone 3 used to be. It’s easier metabolically, but just as hard neuromuscularly because the speed is still relatively high. At that point, Zone 2 training has to happen in moderation and Zone 1 becomes the bulk of the training volume.
@davelockyer101: For most people, the MAF formula is very conservative, and it’s based on averages which is never a great approach. (For people with lower-than-average heart rates, I’m sure it’s a disaster.) Thankfully, it worked for you, but it’s safe to say you can ignore it from now on. Also, with your thresholds that close, you may want to get a proper gas exchange test done.
