That's why you do Boston, there's plenty of distractions on the course with the nearly unbroken lines of people watching. I actually ran a 1:21 on a tough course back in high school for the half. That was coming off indoor track training when I was doing events down to 200m and jumps, which has me thinking I definitely need to work on speed instead of basically ignoring it in recent years. I'll have to see on that 5k this weekend in Needham, my brother and his fiancee are going to be up so might be entertaining them. He used to run too, but haven't been able to convince him to race with me besides the Manchester road race lately.
That stress fracture thing, bit of an odd place to have one if that's the bone I'm thinking it is, but without a bone scan you won't really know if it's a fracture. Best thing (besides obviously going to a doctor, though chances are they wouldn't be able do much of anything for it anyways) would be to just take it easy a bit and see how it reacts to rest.