I'll say three things here:
1. You asked my opinion. I recommend you run the series your friend mentioned, enjoying a chance to run races easily every week or two without risking injury. Get in some 5ks, 10ks, 10 milers even. Have fun racing. Slow down your longer runs. Then stick to your plan and run a spring half and a fall marathon in 2017. But for goodness' sake pay attention to the advice you're getting here or you will remain forever disappointed and convinced you've underperformed.
2. The tendency among runners to qualify their PRs based on course, weather, erratic pace, etc is one of my Letsrun pet peeves. People do it with elite performances here, too. "That 3:51 he ran was clearly worth a sub 3:50 if he had run a quicker first lap." No, that 3:51 is worth a 3:51. While I think you could have run under 3:00 in NYC if you had trained more sensibly in the weeks running up to it, you didn't. I know you haven't called yourself a 2:55 runner or anything like that, but be wary of a tendency to clarify what you've run---I ran 18:40 but on a hot day on a hilly course. Well, okay, but sometimes people run faster rolling through the hills. And maybe you were fitter in mid-September, but your 5k PR remains what you ran in July. Until you run faster.
3. If you read around on these boards, you'll see that the criticism and nastiness you've received is very, very tame compared to some of the abuse hurled around here. I'm not saying that makes it okay. I'm just saying that I don't think you have much cause for complaint. I don't think you're dumb. I do think running with a 121 heat index is dumb.