Here is my pace chart from Boston. The bars show my speed in every 5k/3.1mi segment. The last segment is only 2.2k/1.3 mi long. The differences are accentuated because the X axis starts at 6.8mph. The yellow graph shows the moving average. 8mph translates to 3:16:30. I did 3:22:21. I dropped below 8 at 16 miles (2 hours).
As you can see my speed first increased and then dropped as the race progressed. The much lower speeds in bins 6 and 7 are due to the infamous Newton hills. Without those hills I probably would gone smoothly from 7.81 to 7.53, without a dip. Still, the decline is real. I attribute it to the fact that I did not include any long runs in my training. The longest run I did this year, prior to Boston, was at 13.4 miles.
Ideally speaking one would need a flat graph. Some people that ended up running a 3:16 ran even with me in the beginning, but they kept their pace to the end.
I do remind the reader that my objective is not to run marathons. I use marathons to train for ironman races. If I really trained for the marathon I would probably be able to hold my speed better. That is why I think I could run a 3:15. But it would take a whole lot more training -especially long runs- to do that. Right now, this is training for Lake Placid in July.