I would actually get out faster in the first 5K so that you're not fighting gravity the whole way. I know that's against the conventional wisdom, but it worked for me (the heat not withstanding).
The best way I can describe to run downhill is not to fight it. Let gravity pull you along. The more you fight it, the more you end up braking and eccentrically loading your quads. That's what ends up blowing them out at the end of the race.
At Boston you need to run an even effort, not an even pace. It is a fast course if it is run correctly. But most people don't run it correctly. They listen to the same stuff about "don't go out too fast," blah, blah, blah. That's not really good advice there. Because now you're holding back and breaking on the downhills and trashing your quads. So the right way to approach it is to go out fast on the downhills, and slow down on the uphills (not only in Newton, but on every one of them on the course) and run an even physical effort throughout the first 20-21 miles. Then it's give it all you have left for the last 5-6.