I have never run Boston, but I have coached a few successful runners on the course. The key to running Boston is to run perfectly even mile splits!
This is completely bizarre if you think about it. One would think that he should run by effort... i.e. if you think you're in 6:00 per mile shape on a flat course, the first 17 miles or so are downhill, so run by feel. That may mean that your first mile is 5:45 and your first half is 1:13. The logic? 1:13 first half FEELS like a 1:18:45 on a flat course, and you're going to need that time when you hit the hills later in the course.
This logic is sound logic for a course where there are a few hills (like NYC. Run by effort on the bridges, not time, because it will net out even). In Boston, though, there is too much downhill for too long for it to make sense.
It sounds strange, but if you want to run 6:00 pace, or roughly 18:30/5k, then run your first 5k in 18:30. The effort won't be 6:00 pace. The effort is probably more like 7;00 pace!
But you won't beat up your quads NEARLY as bad and when you hit heartbreak and the long 2 mile uphill stretch late, you will be able to maintain that 6:00/mile clip.
Look at past results. The people who performed optimally ran remarkably even. Look at the 5k split and think. For a guy running 2:35, let's say, 18:30 5k and 37:00 10k DOWNHILL seems like a poor strategy. It ends up being the best strategy! The guys who do that, and are in 2:35 shape, run 2:35. The ones who run by feel end up running 2:40+!!!