As he put it, The symptoms which are considered to be of value in strategic and tactical operations include the following: fits or seizures, dizziness, fear, panic, hysteria, hallucinations, migraine, delirium, extreme depression, notions of hopelessness, lack of initiative to do even simple things, suicidal mania. Greene drew up a list of chemicals to investigate, ranging from barbiturates to carbon monoxide, and he urged a deeper inquiry into the psychological effects of nerve gas
Brain Research Reviews 48 (3): 420437
Driven by the "wellness" trend and smoking cessation needs
There is no backstory to why he smokes cigarettes while running, but it presumably has something to do with nicotine acting as a stimulant