Quick response - New Year’s Festivities are about to commence…
Long hours of Z1 are very good for improving efficiency (a key performance determinant) and also have some cardiac and vascular adaptive inputs. But, you are correct that they take a significantly larger amount of time investment. Not sure about the aging piece ‘over-riding’ the fitness piece - if it did I’d say you had a poor training plan.
HIIT training has a profound adaptive response, but is unsustainable if over utilized… fatigue is too great.
Both high-intensity and long-duration training activate PGC-1alpha, but through different mechanisms (AMPK for HIIT, CaMK for long duration) with similar outcomes physiologically. This is why mixed, appropriately prescribed (intensity and duration) is key for positive adaptations.
I’ll plug Paul Laursen and Martin Buchheit’s new book and course www.hiitscience.com as a great resource for all of this. I haven’t received a copy of the book yet, but have seen Paul (another Canadian, now living in Revelstoke) present on a bunch of these concepts and base a lot of my teaching on their HIIT papers. Paul has another review article on high-intensity vs. high-volume training that is excellent as well.