In reply to ow arm:
Not so much advice as a few observations or things to think about -
I'm a middle of the pack runner with no particular talent, have finished UTLD 50, Old County Tops, UTLD 100 and done a lot of 30-50 mile days so probably living in a world similar to yours
As with all sports training there are some general rules, many of them contradictory and different things work for different people. I know two very successful (internationally) endurance athletes one of whom concentrates entirely on quality with lots of gym work and almost no long training runs and the other does traditional mega miles and 30 hours of training a week.
To me, if you're struggling getting the miles in I would go for really hard work in the gym with a few recovery runs for miles and then some infrequent really long days out and at least a couple of nights out - these are essential
Working out which shoes and other kit work for you is really important and the apparent vast amount of time you have to do this in disappears very quickly. Don't underestimate the catastrophe of getting too cold. Think carefully about opportunities for sock changes and foot care en route. Vaseline is your friend. If there is a spot that always goes try protective kinesio tape on it on long training runs
Try every possible nutrition option - you want to be able to eat their food
rather than carry it but you need to know what you can and can't deal with. I get on great with gels but regard them as backup because I have to carry them myself - again, try the brands
Stuart Mills and Andy Mouncey have superb websites with lots of good advice and inspiration. I take it you have the "bible" Relentless Forward Progress"?
Always concentrate on nothing bigger than getting to the next checkpoint well fed and watered and in decent nick
I think in the end the single most important thing is simply how much you want to finish, not to fail - manipulate your own psychology and find that inner voice that won't let you stop
Good luck, P