I disagree that if you can ride 50 you can ride 100. That may be true for some, but it wasn't for me. When I was training for my first century and building up long miles, the day I rode 65 miles miserably was the day I decided I was NOT going to make myself ride 100 in two or three weeks. Instead, I did the metric century and had a much more pleasant ride than I would have had I forced myself to do 100. I DID, however, do a different century just a few weeks later. I was more ready for it by then, plus that 65 mile ride of misery forced me to get a professional bike fit, which helped a LOT.

I do believe spinning classes can be a great benefit to training for a century, especially if you can't get out on the roads just yet. And it sounds like you are getting in some good rides on the weekends, and your long rides on your actual bike are probably the most important part. Even if you could ride your bike all week, you wouldn't be riding super long distances. Maybe an hour or maybe two. So spinning during the week, without killing yourself "every" time, with longer and longer rides on your bike on the weekend sounds reasonable to me. Just make sure you get reasonable recovery in between each. Good luck! I'm sure you'll do great!