Welshgje give yourself a pat on the back and stop being so hard on yourself.
Take a step back just for a moment and imagine it was someone else who had gone through eveything that you have gone through in the last 16 weeks. Would you be telling them to be disappointed with their time and not to enjoy the fact and be proud that they had just completed their first marathon.
I think you have done BRILLIANT.
No wonder you suffered at the end with the lack of training, the fact that you knuckeled down and finished is what it is all about.
So you know now you can do the distance. When is the next one.
Personally I wouldnt jump straight in and do an Autumn one. Build a good base and go for next year.
Looking forward to reading your account of the day.