Even assuming improvements at the same rate, there has been a drop of  21 seconds in three years meaning it will take another 30 years to break the two-hour mark. See the progression of marathon world best times, where you can see that reducing the record from 2:16 to 2:12 took seven years, 2:12 to 2:08 took 19 years, and from 2:08 to the current mark took another 27 years. 

Using scientific analysis of actual performance trends, Professor Francois Peronnet at the University of Montreal has calculated that the first sub two-hour marathon will be run in the year 2028. 

We have had a poll online asking which world athletics benchmark will be broken next, the 100m in 9.5 seconds or a marathon in 2 hours? The marathon is currently leading the race, though 7 percent of respondents think neither will reach those marks. It may be true that many of us will not live to see those times beaten, though I think one day they will. These records tend to have significant and unexpected jumps in improvement, we just have to wait for the time a sporting freak chances upon the right training and nutrition, prepares for the event perfectly and runs under ideal conditions on a flat and fast course with good pacemakers. That won’t happen very often.

Men’s Marathon World Records

100m World Records (men and women)