Good luck, Frances! What made you choose an Ultra as your first big challenge? You may want to set smaller goals along the way, just to keep the momentum up. Looking forward to reading more about your adventures.
I like to have music for longer runs to figh monotony, evening runs (with caution!) when I can't see much and need some entertainment. I have also used music during the final 10k of a marathon where I needed the extra energy coming from a good beat. I do not use music for forest/trail runs, I see no need as I want to fully immerse myself in nature.
I had similar worries before my first marathon. But the fantastic race atmosphere, the crowd support, the adrenaline in my blood helped me stay strong and focused. The surreal thing is that 4 or 5 hours of running in one go sounds like an eternity but when you are actually running this long, you seem to enter a different dimension and sense time differently. At least that was the case with me...
I have done traditional Fitness Pilates classes and I find that this is best for the core (including abs and back), glutes and general posture. I have tried Ilyengar Yoga, too and certain poses are a a really good stretch (for flexibility). My favourite yoga person is Rodney Yee - he has several really good DVD's for all levels. I would also recommend that you try a gym class called Body Balance - it is a combination of thai chi, yoga and pilates - 3 in one