In reply to CrushUnit:
It should be doable on a standard marathon training volume. You've got bags of time.
1700m over 55km isn't enormous, but is definitely hilly. Get plenty of practice smashing your quads on descents - sounds like you're in a good spot for that. You will probably be out for about 7-8 hours I guess? Thats not too long for an ultra so you should be able to get out for a similar period of time in training. Practice eating on the run with the kind of foods you'll have on race day.
The golden rule is don't neglect the long run. Try and get out for 5 hours or so every 2-3 weeks at the weekend at an easy pace.
Finally, don't take advice off the internet to heart
Be prepared to adapt and be pragmatic, and do what works for you. Listen to your body and take your rest seriously.
I found the book 'Relentless Forward Progress' by Byron Powell quite a good read when I was preparing for my first ultra. You might find that a more structured approach as opposed to a jumble of contradictory stuff on the internet.
Good luck - I'm lucky to live and train in the mountains as well so find it easy to get the kind of specificity needed for ultras on everyday runs.
Good luck.