This chart presents the average time from the gun until you cross the start line based on your finish time. This data was obtained from five chip timed marathons. The chip timing system captures the official start time, and the time each participant crosses the start (and eventually finish) line. The times on this chart are the difference between the gun time, and the time the participant crossed the start.

It is interesting the note the similarity in the start delays even though the size of the fields varied from 5000 to 17000 runners. Another tidbit not shown on this chart is if you throw out the last 20 runners in these fields, all the marathons clear their chutes in about 7 minutes. The last runners were apparently using various facilities at the time the gun went off, since they were 5 to 10 minutes behind the rest of the field.

The effects of the crowds will slow the pace for the first few miles. You can expect to add about 65% of the start delay for the first mile and 35% for the second mile if you wish to run without weaving back and forth through the crowds. For example, if the start delay for your estimated finish time is one minute, you can expect to add about 40 seconds to your first mile pace, and 20 seconds to your second mile pace.