Ambitious!! But you're young and...well lets just stay at young! I think, personally, I'd not be bothering with any road marathons and be looking to get out, off road preferably in the hills or coastal paths. LDWA events are good. Then I'd look to see what other ultras there are in your vicinity, I think there are a few. Even just going out over weekends etc to do long distance footpaths such as Ridgeway. I'd keep off the roads as much as you can as it may be gentler on your joints etc as you have such a steep adaption/fitness curve to follow. I'm sure there'll be others along in a bit to give thier thoughst!!? GOOD LUCK!!
I wish I'd had your blinkered self-belief when I was your age, or even now!! (...thats a good thing btw!)