One thing I always tell my new running friends who are training for their first marathon. Sign up for another marathon right away. Chances are you may not have a great experience in your first marathon, but you will learn a ton. You need to jump back in and give it another try.
I fully admit that I said NO MORE MARATHONS after my first. I also turned into a whiney baby at mile 20. I also swore that I would never run a marathon again. I hated it that much. Then a bit later I did another. I took off 50 minutes from the first marathon to my 2nd and I was hooked. After I ran my 2nd I decided I wanted to qualify for Boston. Why not? At this point, I already knew everything about running. (I am being sarcastic here).
The point is, if I would have just quit at my first marathon I would have never run 19 more and I would have missed out on all the great experiences I had running the marathon but also training for the marathon.