Cheers. Yeah it's pretty daunting. But yeah structure is something I need to do. At the minute I'm mostly doing FTP intervals. They have a few workouts for climbing gains.
I've looked at a few 'how to cycle 100 miles' training plans so probably going to follow one of them, but cover some of the climbs from the event.
You don't need a plan for riding 100 miles. Just get out and ride your bike. It's pretty easy to jump from 50-60 to 100 (I did 70 to 130 last year). It almost always comes down to food and hydration above everything else.
So use your weekends to figure out what your body likes and doesn't like. Asking what people like to eat/drink will almost always result in 100 different answers so it's best to experiment. Myself, and I think Psycho too, enjoy the Feed Zone Portables cookbook. They're easy to make recipes that can make more than enough servings for 100 mile bike ride.
Remember though that just because it's be 30 miles and you need to eat doesn't mean to sit down for a burger and fries. It's always better to slowly eat over the distance (again this is up to you on time, but I try to eat every hour) than to inhale 600 calories at random points. Your body can only digest so much food in a given hour and if you give it more than that you'll get bloating or worse, the runs.
Oh yeah, hydration. Some people swear by water, others swear by sports drinks (not gatorade!) like Scratch or Nuun. Again, figure out the balance your body likes.