28 years ago I got to OTQ marathon shape doing more or less what you say. I followed no specific schedule and pretty much ran the way I felt. Once each week I did a long run with a group that contained some fast guys, and in the pre-Internet era I never knew what route would be planned until I showed up. It might be 12 miles on flat terrain or it might be 22 miles of hills. Whatever it was, I ran it and tried to keep up with the fast guys. I did consciously taper in the last week or two before a marathon. It worked pretty well for me, but I suspect I could have run better with more structured training.