Hi Mr P
IMHO you need to be around 1:20 for half mara or sub-3:00 for full mara to have a fighting chance - and be strong on hills / multi-terrain. Like any race it depends on the weather - and how much ploughed field, gates to climb, etc, have to be navigated, which varies a bit from year to year.
But there are plenty in the 80-90% of RTT runners, like yours truly ,who have no chance of beating the train (the first one, anyway) but still have a great time and keep coming back for more.