I reckon you should seek advice from a professional rather than asking strangers on the internet. You've got no idea who anybody on here is or what their experience is, nor do they know much about your background. It's too easy to follow the advice you like the sound of but really you've got no idea if that's going to be suitable for you. Completing the challenge obviously means a lot to you, so spending whatever on a consultation with a personal trainer has got to be money well spent.
I would echo what Peronel said, spend the next few months finding answers to those questions and you'll be giving yourself the best chance. In addition to that I'd say you need to get your VO2 max as high as you can, being physically fit will make it so much easier. You really want to avoid injury too though so do lots of different things (gym?) rather than just fast running. Again, a PT will know much more about this and should be able to give you a plan to follow.
Range of distances with 115k/+6800m being the big one. It's not had the publicity or big name sponsorship of the two mentioned in this thread but looks every bit as good, and means you might actually get to run some of it too (rather than the conga lines of Transvulcania).
I'm gonna see how the UTMB ballot goes and make a decision after that, looks like a great race though.
Cheers for the Etna link DE, I'll be sure to check that out too.
Your best option is bus to aylesbury and then another one to tring (or carry on to ivinghoe and walk up the road to the beacon, ~15 mins), or if you're short on time a taxi. The aylesbury bus is 280 I think, but takes about 80 mins, though I think they start pretty early so you should be able to make it.