I agree with the chief, not a huge issue. I have done my 3 marathons in Adidas Adios Boosts, which are really similar to the Bostons (same drop, same upper, almost identical weight, slightly smaller toe box than the Boston (I run some tempo efforts in Bostons)). They worked just fine for me, but are maybe a little less shoe than most would want in the marathon - the Bostons are a good bet. Lots of people race in the Kinvaras too, which I am thinking might be the better shoe for you to race in. Both are billed as lightweight trainers, but my sense is the Kinvara may be just a little more substantial.
As to the other question regarding volume: if you are otherwise getting new shoes, I would probably not put a ton of miles on them. I wore two different pairs of Adios Boosts in my 3 marathons. When I got the second pair, doing one long run at marathon pace and one local race was really enough for me to feel that they were adequately broken in (in part because the shoe was identical to what I had worn without issue in prior races). Given you will wear them plenty in the future, I am not sure that there is a good reason to NOT get a new pair about a month out and get just a few runs in them and then feel cushioned and fresh and ready to go on race day.