This was probably on a day with traffic and stoplights and such. Those can really slow you down. It’d be fun to try it someday with all the streets blocked off to really see what you could do.
Not long after I moved to Boston I had a place just a few block from the marathon route (very close to the midpoint, too). I didn’t really know the area well and I wanted to see where the course went so I got a map and headed out on my bike. Rode to the start, then rode the whole course, then rode home. Not a bad day. Don’t know what my time was.
Heard about a group of cyclists that used to ride the course the night before the race. They’d take a train from downtown out to Hopkinton and ride back. Sounded like fun, but I only heard about it the year after the bombing and it sounded like the organizers were really trying to discourage them from doing it.
Actually, it was on a bike path in the western burbs of Chicago. Old RR right-of-ways, mainly crushed gravel. So there are some road crossings, but really pretty clear cycling.
In my experience, as a reasonably fit middle-aged guy recreationally riding a cross bike, 13-15 MPH is a pretty decent pace to sustain.
My usual ride is mostly on a paved bike trail. Those road crossings can add up (slow down, stop, get back up to speed), and the gravel would probably slow me down, too.
My bike computer calculates a rolling average speed, so time spent at a stop is factored out, but the slow down and speed up are included. On a good day I’m still over 15mph (but not by much). If I had the road all to myself I don’t really know what pace I could manage, but it’d be fun to find out. Wind and hills (even gentle ones) would make a big difference.