After hobbling through ten miles of the Oly trials marathon in '92, I decided to quit "cold turkey." (I had a severe metatarsal stress fracture at the time, which made the "decision" relatively easy.) I did little or no running for the next five or six years. When I returned to running, I quickly upped my training quantity and intensity, and got very serious about it again for about a year, but was a bit discouraged by the results, which were decent for a "master," but not what I expected. (Apparently, age makes a difference.)
For me, the difficulty is in getting comfortable with a moderate level of running or other exercising. (Grete Waitz used to say that, for her, it had to be all or nothing.) I remember talking to Ron Dawes ("The Self-Made Olympian") about all of this shortly before his untimely death. At the time, he was really enthusiastic about cross-country skiing, because you could totally trash yourself in training without getting injured. (Actually, he said that you could "kill yourself" in training without getting injured, but his words took on an unfortunate irony a few months later.) Typical attitude for a life-long hard-core endurance athlete.