PG3 has it on the nail - there is a sweep car at Abingdon but a significant portion of the course is on paths where the sweep car can't go. So I can check those portions for runners in ditches or whatever, and the marshals know that when they see me go through on the 2nd loop they are officially free to go off duty.
Hopefully not too many of you will see me at Abingdon, as I've volunteered as tail cyclist (though I will be marshalling in the first mile too) (anyone want to marshall? We could always do with a few more!)
I will be tail cyclist this year. Have just come back from a chat with the RD, who was clear that the 5h limit would be enforced somewhat more firmly this year. Nothing insane like the Comrades finish gun, so BadgerBoy don't start panicking!
I'm sure people have done the marathon with a training background like that when it wasn't their first, and they had (in the longer term) the training base and experience to sort of muddle through.
But your first? It might be a thought to look for a marathon you really like the sound of, next spring (or even at the turn of the year if you can afford it), book in for that, and then just have a nice weekend in Dublin but focus on recovery.
Alternatively you could follow the physio's advice for getting to the start line *healthy*, crosstrain as you can but don't go mad, throw a walk break in every five minutes and reduce your expectations as much as you can.
I've had similar experiences myself so I know marathons *can* be finished with pretty awful prep but it is SOOOO much more pleasant if you're healthy and have got solid training in.