Claire, Seems like we are in a similar position, although you have done more marathons than me!
I decided that this one would be better than the High Peak 40, as it will be flatter - although 10 miles longer! My view is that you are going to run that sort of distance, get the most bang for your buck!
Anyway, I have been researching training etc and the golden rule seems to be run lots of long runs (possibly back to back) and learn to walk at times. Most seem to think that a run to walk ratio of about 5:1, so training with this idea on long runs helps. Also, learn to eat along the way. My goal time is about 9 hours for 50 miles so even if not running you need to eat in that time, so again preparing for that in training is key.
I am doing the Stafford half at the end of March then will be concentrating all running efforts on this run. I find the whole thing a little daunting but hey, whats life about if its not doing mad things like this?
Rob