With 50,000 runners the race starts in stages or waves. The professional runners start at 8:40 and all other runners start at 5 different times starting between 9:10 and 11:30. The times are very punctual. Based on the early start the corwds start gathering at about 9:00 on 4'th Avenue in Brooklyn and a bit later as the route proceeds north.
There are people all along the route. Some areas have more specators than others. See the maps in the Route menu for more information.