Haven't really read much of whats been written so apologies if its a regurgitation. If it motivates you then go for it, you've got time to train.
I've done alot of running - fell, trail, road and ultra marathons.
First thing to be aware of is not upping your mileage excessively especially if you are new to running. Noakes Lore of running quotes 10% increase in weekly mileage which is generally the rule of thumb.
Physically any one capable of running a half marathon could probably run a ultra without much extra preparation, the main thing comes down to the mental aspect. No matter how much training you do its going to hurt and its down to how mentally tough you are and your will to finish the race.
The second thing to consider is pacing, ultra races unsuprisingly aren't particularly fast you just need to keep tapping out the miles. People new to them tend to go off too fast in the first half of the race suffer around the half way mark.
Next up, is feeding and hydration. You need to get this nailed in training. With practice you'll realise when you hit energy lows, and need to hydrate more but this only comes through practice and sometimes you'll still get this wrong.
Finally I'd make sure you are factoring in some core work in to your weekly routine, perhaps a couple of sessions with a running coach to work on running form and finally factor in some speed work so you don't become a singlespeed plodder which many ultra runners inevitably become.
+1 LDWA events