Zone 2 training can produce big improvements in endurance at constant HR. Is this training the heart or training the leg muscles?
If your Zone 2 is equivalent to 180-Age HR, then that's Phil Maffetone territory; Aerobic base training done at such a low level, you can do massive volume without any risk of injury or over-training.
I spent a long time last year trying to ride a bike at these kinds of HRs - it's really hard, but very gradually, you get faster for the same low HR. I guess my HR is 10-20bpm lower at audax speeds, than it used to be.
It's such easy work - I can't imagine it builds any muscle as such.