Ok... well the principles of training remain. You need to train with the distance and terrain in mind but do not neglect other aspects of your running.
NDW for example. 50 miles of hilly trail. It is ideal that you are close to it so for your long runs then getting out on to the terrain is ideal. Taking a marathon training plain you can pretty much maintain it with the addition of back to back runs when you feel able to i.e 25 Sat 15 Sunday. With this extra mileage in mind keep an eye on your recovery, keep mileage up with easy recovery runs 60 - 90 mins for example but without stressing your legs much.
A lot of people have ultra runners down as super slow plodders. I am a big believer in maintaining speed work in my training for various reasons. Enforce good running form, to provide variety in my training, to improve dynamic flexibilty (a fast run will put your muscles through a wider range of movement than a slow run), to improve lung function and to improve how you use lactate. Obviously you will not need the speed of Mo Farah but try to include tempo runs, intervals etc at least once a week. Ideally midweek if your long runs are at the weekend to provide recovery time.
Babbling a bit but hope that helps!
