IMHO, running a marathon has nothing to do w/ running an IM - they are simply 2 different sports. So, your question "would it be good for training" is answered by a definite no, I believe. This is the reason you don't see top IM triathletes running marathons "for training."
However, it would seem to me that imbedded in your question is "can I run Boston and still have a successful IM" and the answer to that is definitely yes, but be sure you understand that is a different question.
There was a thread on RecSportTri as to whether triathlon is one sport or three. I think it is definitely one; so, training for a marathon is a separate sport than a triathlon. Of course there is some cross over fitness, but what is "good for training for an IM" IS training for an IM, not a marathon.
Hope this helps!
David
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