I think John L Parker nails it’s simply. Run you short runs hard (z4 Karvonen) and your long runs slow (z2). Avoid the middle ground.
After many years of triathlon I got bored with the standard progression through the distances and the rising cost, so started ultras and other unusual endurance events. Ran several 50 milers and a couple of 100 XC which were really mental more than physical challenges. But there was a mistake in that you lose your speed endurance badly. I struggle to run short hard runs now eg 40 mins at 170+ and bizarely can run hills but hate long steady flats.
Done three XC runs (2 x 60m and today just 45) and a bike ride this week and shocked by the lack of basic speed. Nothing in the legs at all.
Time for some short, hard stuff and hill intervals. Oh and some weight loss!