In reply to Walshy: When I did my second marathon, the first London Marathon, it was also my first in under three hours.
I had to walk backwards down the steps to get to the tube, It is not like fell running, it is a far less varied gait, eventually I got better at the aftermath of them, but if you race it, ie go for a time, expect to suffer for at least your first couple, I then got better at recovery, but I still always found the White Peak Marathon on the old railway tracks less damaging than road running.