Brighton marathon this year featured a lady running alone in front of an ambulance showing blue lights when everyone else had cleared the course to make way. Yes, she had headphones in so couldn't hear. Fortunately it wasn't a life or death emergency and someone pulled her to the side but it shows that there are people who plug themselves in and ignore everything going on around them, which can affect safety.
Brighton marathon's rule is that headphones aren't allowed. They don't enforce it but state that if there is an incident that is the result of, or made worse by, you wearing headphones you will be held liable.
I ran a negative split at Brighton this year, by 10 seconds. You have to know exactly what you're capable of (not what you imagine you could do) and stick rigidly to your schedule in the first half. You can't think "I'm going really well so I'll speed up a bit" as it will bite you on the arse later. It's also easier on some courses.
The reason I managed it was because I was undertrained so ran a conservative race. I was confident of hitting 4:10 so that's what I aimed for, rather than trying for a hopeful 4:00.
It has been known for people to run with a second chip, so get someone else a GFA or an entry to Boston. However, it's a simple matter to spot them in the results as you just find two people who have crossed every mat at exactly the same time. Then you check the photos.