Hi newbie! I'm a newbie, too. I just started up cycling again a couple months ago, and was only able to go a few miles without pooping out at first, didn't know how to shift my gears, had never clipped in before, had never ridden in a group, knew nothing about nutrition for longer rides, and my co-workers had already signed me up for a 165-mile, two-day charity ride in November. Here's what I did:

1. Read every thread here and on a couple other forums.
2. Bought a nice bike and had the guys at my lbs show me how to shift
3. Looked into and contacted a local cycling club (hope to ride with them in about two weeks)
4. Contacted a local cycling performance place (use computrainers), and took a private bike handling lesson in a parking lot
5. Took a basic maintenance class at another lbs
6. Roped two friends into riding with me (they are now totally into it)
7, Bought a bunch of books on endurance cycling. My fav is The Time Crunched Cyclist, which gives you a training program, even for complete beginners, to complete a century
8. Bought a cyclops trainer for my home to do intervals, which I really can't do on the only safe road to ride around here due to lights, traffic, etc.

Right now, I can already do about 50 miles at about 15-17 miles an hour, but I'm hoping to be able to be up to about 18 by the time of the ride. Although this might not be for you, I was able to hire a coach and work with the performance cycling place, signing up for a six-week power training class (two days a week), where they basically measure your thresholds at the beginning, and set a program that's individualized to your level (yes, I'm the worst in the class) on a computerized trainer, work on pedaling, etc. I've only done three classes so far, and already see improvements on my once-a-week long ride (feel stronger, using the whole pedal stroke, rather than just pushing down, am faster). My coach, a woman, also signed me up for an all female, no-drop group ride on Monday nights, and signed me up for a metric century in July. She enters my weekly plan (rides, weight training-i do this already) in an online site called Training Peaks. Plus, I type in comments after each ride, plus enter in everything I eat, and she enters comments, and adds things to work on, etc. It's a free program that you can enter your own plan in if you like (if you find one you like from a book, for example), and they also have programs/plans you can purchase that basically tell you want to do and when to do it. Just type in "Century Rides" and multiple choices pop up, though they seem a bit expensive.

Right now my weekly plan looks like this:

Two high intensity hour-long rides a week (on the computrainer)
One social hour-long group ride
One one-hour long slow ride (heart rate 125ish)
One long ride (right now that's 40-50 miles, but will be increasing)
Two weight training sessions (I'd been doing this prior to getting a bike)

I know that it seems like a lot of time, and if you'd told me two months ago, I'd be doing that, I'd say you were crazy, and that I couldn't possibly fit that in my schedule, but I'm having so much fun, I look forward to it, and have made time.

The group that I'll be riding with on my event (not a named group, but dedicated locals that do this charity ride every year to support local HIV/AIDs support and outreach centers), has group rides starting this week to bring everyone up to speed, starting with 23-mile rides. Since you mentioned you're riding with a group, maybe there is someone you can contact there and see if there is something like that for you.

I know it's been said before, but if I can do this, you definitely can. I'm in my 50s, out of shape, and all I can say is that I'm really having fun, have already met some wonderful new friends, am enjoying hanging out with old friends in a new way, feel better and have even lost 10 lbs so far (30 more to go!). I don't think I'd attempt 180 in one day, but if it's a two-day ride, and you have a few months (hopefully at least three), and you get into it, it's doable. Worse case scenario is you just learn to enjoy getting out there riding, and save the big rides for next year. Have fun!