Walking for the first three miles? I refuse to believe that it's that badly organised. Yes, I've run in events that size and while you're walking to get to the start line and the first few hundred metres are pretty jammed up, I've never run in an event where you have to walk any part due to crowd congestion other than the finish at the Sporting Life 10K in Toronto, which is less of a race and more of a fun run.
Maybe the non-qualifiers at the back of the Boston field jam up a bit, but I refuse to believe that a field of experienced marathon runners, the vast majority of whom have had to run sub 3:30 just to qualify, would be walking for three miles at the start due to congestion.