This guy did it in 4:34:12 and his Flat Marathon average is 3:15.
He suggests you add around 40% onto your flat time. The averages show that most people go out too fast. Based on this I will be holding back by around 10% in the first 26km compared to a flat Marathon.
I think you can expect to be 1hr - 1hr 30 slower over the whole distance but the vast majority of this difference will be in the second half so you shouldn't have to change your usual marathon pace for the first half. I did London in 3:19 this year so I'm aiming for a sub 5 hour.
Im not sure what the friday registration is but i'm guessing its something similar to London, where you get your number etc.
There's around 8000 runners over the 2 days with 4000 expected to run on each day.
Don't worry about the second half, most people adopt a run walk strategy when the incline starts and once you hit the really steep stuff its like one long conga line of fast hiking. One thing you can do is leg strength work. Do some squats and lunges a couple of times a week.
Im in for this too. Just done a very hilly Marathon in Lancaster(Lancashire) as preparation. My wifes Uncle has done this every year for at least 10 years and he is primarliy a fell runner. His advice is to stick to your usual Marathon plan as far as milage is concerned but do at least 2 of your weekly runs as hill sessions. Ive also been doing every other LSR on hills too.