It's rare but you should be able to negative split unless there's a strong headwind off the ocean in the last six miles. I was able to negative split my first Beantown back in '98. My friend and I went out in about 6:50 miles. Fortunately he was the one who had run it before and kept saying "just hold back, they're all gonna pay for it." At about mile 2, two jerks, one with a lot of tatoos and the other with huge dreadlocks shove between us and yell "get out of our way we're trying to run fast here!" It really pissed me off, but we stuck to our pace. I believe we passed the halfway in around 1hr. 28 min. Well I eventually felt good enough that I started to pick up my pace, and by the time I hit about the second incline around mile 17 or 18 who do I catch up with but our two buddies absolutely dying (I probably wouldn't have recognized them but for their distinctive appearance). One of my most satisfying moments in running. My friend who passed them a bit later had the wits to say "out of the way we're trying to run fast here." I finished off the last 10k in about 40 flat and ended up a little over 2hr. 53 min. I don't think I've come close to negative splitting in the other five Beantowns I've done, but I'll remember that one.