I use HRM and turbo during winter and for some interval training all year round For a reality check wear your HRM say at local trail centre and record your time and zones. My HR for the black at Afan are nearly all 4 and 5. Little or nothing in Z2. My maximum and rest have been tested professionally so I know Im set up correctly. Point is that for MTB you do need endurance but that could mean using Z4 and Z5 for say a 4 – 5 hour enduro. I tend to just use Z2 for recovery but most of my 3 – 4 turbos per week this time of year are aimed at developing my threshold. I will add distance when the weather warms up 🙂 Just dont be too fixated on long slow. Whats your goal? Adapt your training to your goal. Lots of Z2 may well have a negative affect on top speed or burst power up a steep incline.
I think more in terms of training that brings about an adaption of my VO2 that raises my lactic threshold. So on an enduro I may be in zone 2 but they may well allow me to maintain a lot higher speed/effort than others. One persons threshold isnt the same as the next and its that threshold that defines your ultimate endurance. The higher the better. The change from fat to sugar burning is gradual and this explains why very fit people need less water and food during events as they simply dont burn it up at the same rate as a less fit person for a given speed/effort.
Sorry for rambling but wanted to make sure you noted the plus and minus of long and slow training and alternatives approaches