Charlesvdw
The smaller local marathons I participated in have a 5 hour or 5:15 cut-off.
And no pace groups slower than 4:30.
Boston, the only major I have done, closes after 6 hours.
Even a person jogging slowly should finish in 5 hours. Longer than 5 hours mean frequent walk breaks.
If one has to take walk breaks, one has to train harder for the marathon or stick to turkey trots.
Besides, the London marathon has to reject tens of thousands of runners each year, they should NOT give spots to 6 hour pedestrians.