How can runners predict a finish time halfway through the race?
The short answer, after analyzing thousands of finishers splits over the years: multiple your 13.1 mile time by 2.1. Cross the half-way point in 1:30, you could finish in 3:09, for example.
But first, note that statistically, the 2.1 multiplier is a decent guide. But it won't hold for those who started too fast, or finished with extra energy left over.
The blue dots in this chart plot the intersection of first and second half splits for each Boston Marathon 2013 finisher. The cluster around the yellow trend line hold reasonably true to the finish multiplier of 2.1 times half-marathon split. But you can also see a great many runners who blew up in the second half of the race, shown in the general upper left area
Of course there are variances, and I've got answers for you, by age gender, age group and by 30 minute finish time increments for all who finished under 4:30 in 2013. See age group details below.
For more, see our complete Boston Marathon Race Coverage.