Running author Joe Henderson published the book Long Slow Distance; The Humane Way to Train (1969). Others had also known that a good way to train was to run slowly over a long distance, but Henderson coined the term “long slow distance.” It is often abbreviated “LSD,” with a nod to the then-popular drug LSD (lysergic acid diethylamide).
“LSD (long slow distance)” was cited in print in August 1969.
Wikipedia: Long slow distance
Long slow distance (LSD) is a form of aerobic endurance training in running and cycling. Physiological adaptations to LSD training include improved cardiovascular function, improved thermoregulatory function, improved mitochondrial energy production, increased oxidative capacity of skeletal muscle, and increased utilization of fat for fuel. Ernst van Aaken, a German physician and coach, is generally recognized as the founder of the long slow distance method of endurance training.
Long slow distance training is a form of continuous training performed at a constant pace of low to moderate intensity over an extended distance or duration. The moderate training intensity of LSD is effective in improving endurance and maximum oxygen uptake in individuals who are undertrained or moderately trained. Long slow distance training is thought not to be effective when used in isolation by well-trained athletes, who in order to achieve further improvements in metabolic conditioning require higher training intensities that are not sustainable at the work durations associated with LSD.
Tim Noakes, a professor of exercise and sports science at the University of Cape Town, suggests that it was Arthur Newton who initially proposed that running longer distances at slower paces was the most effective training method for beginning runners. Noakes asserts that after this method was rediscovered in the 1960, Joe Henderson coined the term “long slow distance”.
Long slow distance running was promoted as a training method by Joe Henderson in 1969.
Lysergic acid diethylamide, abbreviated LSD or LSD-25, also known as lysergide (INN) and colloquially as acid, is a semisynthetic psychedelic drug of the ergoline family, well known for its psychological effects which can include altered thinking processes, closed- and open-eye visuals, synesthesia, an altered sense of time and spiritual experiences, as well as for its key role in 1960s counterculture. It is used mainly as an entheogen, recreational drug, and as an agent in psychedelic therapy. LSD is non-addictive, is not known to cause brain damage, and has extremely low toxicity relative to dose. However, acute adverse psychiatric reactions such as anxiety, paranoia, and delusions are possible.
OCLC WorldCat record
Long slow distance; the humane way to train.
Author: Joe Henderson
Publisher: Los Altos, Calif., Tafnews Press 
Edition/Format: Book : English
4 August 1969, Ironwood (MI) Daily Globe, “Paavo Nurmi Marathon” by Vic Godfrey, pg. 10, col. 5:
During this LSD (long slow distance) period of training, no speed work was done.
Guide to Distance Running
By Bob Anderson and Joe Henderson
Mountain View, CA: Runner’s World
It could be called LSD (long slow distance) with a German accent.
30 July 1973, Boston (MA) Herald American, “Keeping Fit by Proxy” by P. Albert Duhamel, pg. 11, col. 1:
If Bill Proxmire had his way, he’d have the whole country on LSD. The senator from Wisconsin thinks everybody should keep fit by Long Slow, Distance-running.
The Complete Runner
By Bob Anderson
Mountain View, CA: World Publications
Slow Distance— This is the “LSD” (long slow distance) popularized recently by Ernst van Aaken, Tom Osler, Joe Henderson and others.
Running A to Z:
An Encyclopedia for the Thoughtful Runner
B Joe Henderson
Brattleboro, VT: S. Greene Press
LSD means Long, Slow Distance to a runner. When that method took its name in the 1960s, ...
Diary of a Word Nerd
A Runner’s Lexicon
Posted on March 12, 2014 by Julia Tomiak
LSD – Long Slow Distance. Unexpected flashbacks to particularly brutal miles common.
New York City • Exercise/Running/Health Clubs • Thursday, March 13, 2014 • Permalink