Today’s tragic explosions occurred at about 4:09:50 of the official marathon time, according to video of the events.  Though it appears that more people finished the race early this year than last year, the 2012 statistics can provide a rough ballpark of the rate at which people were finishing the race around the time of the explosions.  In 2012, 582 runners finished the race with times of 4:08:00 – 4:10:59, or about 32 runners every 10 seconds (race times may not match up with the official marathon time).  During the 4:08:00, 4:09:00, and 4:10:00 minutes last year, the breakdown was as follows:

Thus, dozens of runners were likely in the vicinity of today’s explosions, as was confirmed by video of the incident.

(The breakdown from this year was not easily accessible.)

Source:  MarathonGuide.com