Some key points...
1) In 2017, three men ran under 50 minutes. In 1987, it was fourteen.
2) The time for the 50th finisher in 2017 was 57m 45s. In 1987, the 50th person was 54m 08s. The time for 50th place in 1987 would have finished in 16th place in 2017.
3) The time for the 100th finisher in 2017 was 59m 50s. In 1987, the 100th person was 56m 46s...roughly 3 minutes faster. The time for 100th place in 1987 would have finished in 42nd place in 2017.
As for why the times were so much faster back then? Basically, it mostly due to the fact that in 1987, most of those in the top 100 were young men who ran a lot. The age profile of the average runner now is older.
The 1987 results can be seen below...