That is oddly enough the same for me. I did two days of Jogging at a pretty slow 9:00-10:00 min mile pace for an hour at a time on the treadmill and then have been able to since the Marathon run on the roads and trail at my usual MAF HR and pace. Surprisingly my pace picked up since the marathon although being at the same HR.
I think for us since it was our first marathon we just either had too low or too high of expectations. That group of female runners that had passed you and talked in the last 10km or so that you mentioned, I ran with a pack of 2 for the last 10km before finishing and they had started at the 3:30 group but caught up to the 3:20 and then felt great from there forward. I believe the two were even qualifying for Boston based off of the paces they were at?
I was able to talk the whole time also, I think this was a good indicator that I should have went harder, but I think like the others had said, we learn and move on. Move on in our training, to the next marathon, etc. but pace ourselves (in our races and training). There isn't a reason to rush the process right?
I'd rather stay injury free and be running for the long haul at high but slow mileage and see what happens, I'd love to sub 3:00 and BQ qualify too like you. But I'm average, and like my name suggest, I'm in it for fun.