Most people pick someone else’s training plan and just follow that to the letter. That’s probably alright for most, though having a life or other complications can make following most plans a problem.
For example, a Hal Higdon marathon plan follows a fairly set schedule. The intermediate plan has cross training Monday, three easily doable runs in a row on Tuesday through Thursday, Friday off, a moderate run Saturday and then the long run Sunday.
What if you run in the evenings after work but have a commitment on Thursday night that interferes with that run? Or what if you run in the mornings before work, but the 7-8 mile Wednesday runs later in the program are too long to do before work?
Or what happens if you’re exhausted and getting sick at the end of a week? Do you risk compounding that problem by getting your workouts in? Do you risk compromising your training by skipping the Saturday run (or heaven forbid, the very important long run)?
Never mind scheduling concerns: What if the weather is blazing hot and doing a 15 mile long run, even early in the morning when it’s cooler, simply is not do-able without risking serious health problems? What if doing the whole run on a treadmill or otherwise indoors just isn’t practical?
Conversely, what if it’s the dead of winter and windchills have dropped to a deadly low, or your locale just got hit with two feet of snow?
A lot of novice runners would just skip every workout that runs into such interference. And most will get to the start line of their goal race woefully undertrained.