That's GREAT!!! Good luck! I've done a two day event that was 100 miles the first day, and 40 the second. I trained kind of like one would for a marathon, because it's very similar.
First and foremost - take your bike to a bike shop and have them fit you correctly. A comfortable saddle is a must! I'd recommend going clipless on the pedals and shoes because it requires less power output over all (it's easier - once you get used to it). And get bike shoes that are about a size too big - you'll really appreciate that on the third day when your feet are swollen and your shoes are not pinching them. Wear bulkier socks the first day (and on your training rides).
Plan on riding three to four days a week. Plan one long ride week - I would actually follow that long ride with a recovery ride the day after, so you get used to being in the saddle two/three days in a row. You'll increase your mileage slowly. Your schedule should look something like:
Day 1: Cross train (endurance weight training or other cardio or optional rest day)
Day 2: Short ride - 10-15 miles (you can start smaller - but to me that's a fairly short ride)
Day 3: Cross Train or rest day
Day 4: Tempo ride - if you have a trainer or a spin bike use that and ride one of the Spinervals workouts or a Cardio Coach (I'd use iCycles sparingly for this - you really want a true interval training here). If you have to ride outside, pick a short loop that's flat and ride sprints for a lap or two, then pull back for a lap to two. This ride is important as it will train you to ride faster over all. Put your heart into it.
Day 5: Rest Day
Day 6: Long ride (start with what is your current long ride - for me that would be about 30 miles - but do what's right for you - you want it to be a challenge, but doable). Add between 3-5 miles a week until you are comfortably riding 60 miles - and I would ride 60 miles two or three times here because you will be doing this more than once. (typically if you were riding 60 miles, I'd tell you to train for 50 - the last ten will come easily during the event). And try to ride terrain similar to that of the ride (if it's hilly - don't train exclusively on flat roads - you'll be sorry).
Day 7: Recovery ride - you can use the spinbike or trainer - or just go out and pedal easily on a flat route. Go for about 30-40 minutes - and keep a higher cadence, lower resistance spin. You do not want to push it. Spinervals has some great recovery rides, if you have a trainer or spin bike.
About 3 weeks out from the event, skip the recovery day and ride another 40-60 miles. This will give you insight as to what to expect when you ride twice in a row - what hurts, where you might need some body glide, if your equipment is comfortable... You'll get even more benefit if you add the recovery ride AFTER the second - three days in the saddle...
Hydrate frequently (a camelpack is a lifesaver on long rides). Make sure you eat something every 1.5 hours or so (not much longer than that) - believe me - the worst thing on a bike is to be out 30/40 miles from home and "bonk" (run out of fuel - but it's like hitting a brick wall - it's not fun!).
Wear bright colors - make sure you can be seen.
And oh yeah - ENJOY! You'll learn more about your neighborhood and surrounding areas than you ever thought possible when you plan distance rides.
Feel free to PM me for more. I love endurance cycling events.
