I ran this last year, No problems with the start. I got to the pens about 20 minutes before the start and got fairly close to the front. I could run the pace I wanted straight away(slower than Mr Cleasby and Joolska).
A couple of tips - avoid the breakfast run on Saturday, it was Borderline dangerous! The pasta party was good, excellent pasta and real beer.
I followed the instructions about taking it easy on the early downhills, so felt almost too comfortable in the 1st half. I slowed down a bit due to the warmth after about mile 11, felt ok going up the hills (apart from one at the fire station), but my legs still seized up going down the far side of Heartbreak Hill.
Agree with Jonah8 about the support. It was my 1st Boston, but I didn't think it was possible to make more noise than Wellesley and Boston Colleges, until I arrived at Fenway/Kenmore. The noise was almost too loud!