In reply to edhawk21:
I'm no expert, but (classic uninformed UKC response pending...)
I do similar mileage, about 100 miles/month, spread over about 5 runs a week (3-4 short, one long weekend run, all off road). Longest so far is about 17 miles, after which my knees were really sore. I'm slowly trying to get my body used to longer runs by getting plenty of shorter mileage in and pushing the comfort threshold higher. It takes a long time though.
So, my question would be: how do you feel after your longer runs? Could you keep going, do the same again, or are you at the comfortable limit of your knees/ankles/hips etc. I'd have thought that getting up to ultra distance without buggering your body quite badly will be a real challenge, but it totally depends on how your body responds and what you feel you're capable of.
I will watch the thread with interest as actual experienced runners give their opinions, as I'd like to get my distance up to marathon too. Speaking of which, why not a normal marathon or less distance? Edale Skyline, 9 edges, or something like that. There are lots of great long, hard running challenges that don't necessarily need to be an ultra. Obviously my interest in entirely trail, not evil roads.... your OP seems to imply you're a road runner.