LSD, as a running term, was invented (probably) and popularized (certainly) by Joe Henderson is his 1969 book, titled Long Slow Distance: The Humane Way to Train. (You can read the entire thing on Mr. Henderson's website, free of charge.)
This was a noteworthy salvo in the war between volume and intensity, a strike against the then popular model of near daily interval training for distance runners, fired before science could explain the physiological benefits of running sub-maximally, before "mitochondria", "capillaries", and "slow twitch" became common parlance in running circles.
Thus it is no surprise that Henderson does not deal in scientific terminology. In fact, his argument is basically this: Work with your body, rather than against it. Training must be consistent to be effective; and to that end, it should be enjoyable and sustainable, both from a physical and psychological standpoint. To quote Lydiard, "Train, don't strain."
"LSD" was a catchy acronym for this philosophy, although "slow" was not a wholly accurate description of what Henderson did and advocated (inasmuch as he advocated for anything). There were plenty of fast miles to be logged by LSDs disciples, simply not at the expense of an immense volume of steady miles, a necessary precondition to holding any kind of fast pace for any meaningful distance.
What the acronym did convey was the sense of "high" that runners enjoy at the best of times, and seek at all other times. Henderson argued that you could benefit from chasing that high, rather than chasing the rabbit, that productivity and pleasure need not be mutually exclusive.
The thing is, if you don't know this already, you probably haven't read this far. If you have made it, you still aren't convinced that cruising along for an hour or two, sitting just below your lactate threshold, could be pleasant - indeed, could be fucking euphoric. And if you know, you know. You're nodding although no words manage to convey even a fraction of the totality and depth of the sensation.