I've done it religiously for 2 winters and had some decent 6/12hr mtb results but my top end work wasn't good enough and races came as a shock.
This winter I've had less time so have been doing 2 interval sessions - one at threshold and one at z4 plus Z2. When I don't have time for z2 I do a shorter z3 session.
Z2 is needed to give a depth to your fitness. It makes physiological changes and allows you to work harder for longer later on.
The danger with too much higher intensity at this stage is not being able to keep it going.