Hi Cath:
This is my 5th season of cycling, love to ride with friends. I picked this ride out very early in the year as a challenge to work up to. I'd ridden the metric distance on this ride (62 miles, with 5,000' of climbing) twice before and loved the course, but wondered if I could actually do a couple thousand more feet of climbing and a longer distance to boot. Also, I'd probably have to ride it by myself, which I wasn't looking forward to. However, there are lots of people along the way to chat or ride with so it really wasn't that bad. I felt very accomplished when I finished this ride and definitely it was everything I wanted in a challenge! I'll be back next year.
I work full time, ride my horse and like to do outdoor stuff with my DH so I really don't like to ride more than 2-3 times a week. That's where my daily Cathe workouts come in. Imax2 and anything with intervals works great to stretch your cardio threshold. Also, someone on here suggested doing lateral strength movements and floor work (GS Legs and kickboxing work great for this). Also any core work is great for cycling as you need a strong core to transfer your power from your legs to the pedals. And of course, any of the weight work (endurance or whatever) just makes you stronger overall. On a weekly basis, I usually do a shorter ride (30 miles) with climbing during the week and a longer ride 50-70 miles with climbing on the weekend. Other cyclists I know ride a lot more than that, but I just don't want to give up any more time for it and it works for me. Spinervals is supposed to be great too.
Mostly, I think, if you can ride 60 to 70 miles on your own, you can ride an organized century. Of course if it's a hilly century and you don't ride hills at home, it will be a struggle. Cycling is a pretty easy activity to progress in. I did my first organized metric (62 miles) after having a road bike for only 3 months and did my first century early the next year.
I'm a big believer in finding a fun exercise activity and having a buddy at your level to do it with. Also, pick organized rides/events to shoot for.
Hope this answers any questions, but if you have more, let me know.
Jeanette