Most training plans really do seem to have three key workouts: intervals/hills, some kind of tempo, and long run. The rest is filler and some include strength training. The mileage builds through the course of the plan up to 2-3 weeks beforehand when it drops off. As long as you are sure to hit those three key workouts and spread them out through the week, yes, you can run a marathon on that.

If you start earlier, you may have the luxury of alternating weeks - not having a long run every weekend, instead taking the time to build slowly. I felt less tired when I did it that way. You still have a long run (or ride even!) on the weekend, but shorter than you'd normally do in your build. You notice it more in the last few weeks - where I'm doing 18, 20, 22, 10, race miles, you'd be doing 18, 10, 20, 12, 22, 14, 10, race sort of thing.

Anyway, yes, you can definitely do what you're talking about, as long as you're dedicated to it. I wouldn't call that half-assed, more like less structured.