You’re probably remembering right, but it’s incomplete advice. Both strength and endurance take a long time to build, especially the structural changes that lead to large improvements. Things like stronger bones, stronger tendons, increased capillary density, increased mitochondrial density – those take a long time for the body to build.
There are some adaptations that are quicker to take effect, and are lost quickly as well. A few sprint workouts can get your nervous system to recruit more of your muscles, and build up some enzymes that will make energy available more rapidly. Those are useful and important – but aren’t a shortcut.
I’m reminded of Lon Kilgore’s book “Fit,” which has a bunch of great ideas about looking at fitness through the lenses of strength, endurance, and mobility. A good balance – but he defines endurance as the ability to run 3-5 miles without stopping. That’s a very, very different approach to “endurance” than you’ll find discussed in “Training for the Uphill Athlete” and on the various articles on this site. Three miles is just a warmup – the aerobic (i.e., endurance) system is just starting to get switched on.