My guess - and I do mean guess - is that a Century in the summer or even Fall (when there are usaully many) would be better. I have known a couple of people who did more than they should have and felt a little turned off to cycling.

I just started cycling a year ago and skipped the Empire State AIDS ride in August for the same reason (It's 560 miles in 7 days) - I love (an understatement) cycling and didn't want to be turned off. So, I'll be doing something like it this year instead and I feel pretty confident now.

I don't know about other riders - but spinning class is not the same as road riding for me - I do take spinning classes in the winter months here - just to keep the same muscle groups active - but climbing a real hill is different for me.

However, if you do decide to do it - It's recommended (from vereything I've read and trainers I know) that you train by riding 5-6 days per week, increasing your mileage by about 10-11% a week, working your way up so that you can ride at least 80 or so miles. Maybe the club you're riding with would have a recommended training schedule for the century.

Since I've only been cycling a year, it will probably be good to see opinions from some more experienced riders on the forum, too. Good Luck!