In reply to Chesher cat:
80k (or 50 miles in old money) is a good distance to start building up your ultra racing but I would advise against forgetting speed work altogther in building up your distance.
If you have completed the MB Marathon before, you've already proved a good level of fitness and maintaining this is key. A lot of people move solely to longer, slower runs and a lot of fitness that they had before is lost. Whilst getting in some longer runs is important (as is some long days trekking up hills as you will be doing a lot of that too) I implore you not to forget your speedwork.
Getting to 80k will be a lot about your pacing, food, drink and body management. You have the fitness to get you there, you just need to adjust pace, look after your feet and get all the food you can in. Longer distances ultras are just eating and drinking competitions with a bit of light running thrown in.
You don't need to be running 120 miles a week to run Ultra marathons, it is just important to concentrate on the quality of the miles you get in, rather than just the quantity.
Lastly, get to some UK ultra events, chat to people and get some advice out there. Much more than just a training run. Look at the LDWA website, plenty of cheap runs with soe very experienced runners.
Hope that helps a little.