Welcome, Since. Great screen name. Nothing trains a rider like riding- you need to be in the saddle every day, well six days a week. Spinning is good, but you need a lot of base miles, or now it's time at a certain intensity. Low intensity. Use a heart rate moniter. The pro's train on rollers in the winter, in front of the TV. You could go out in the dark and cold for a few hours a night, but are you really gonna do that?
Spin class is short and intense, that's good for some aspects, but you need rides of a minimum of two and a half hours several times a week. Rent movies, work up to it. That 2 1/2 hour mark is the training adaptation point. It's a freaky feeling, down the line you'll really feel the change in your body as you go through that time. I got this off some headcase old race mechanic and he was right- just keep the intensity low and the speed will come up over time. Your endurance will increase at the same time. It's real grunt work but it pays off. You'll need some much longer all day rides, too. Weekends in the country. 60 divided by your cruising speed, at 20 mph thats at least three hours a day in the saddle.
You are gonna look so great in short skirts! Well worth it.
You can start this on the trainer, but rollers are almost a perfect simulation of the road. They work everything and demand the same coordination and proprioception that outside does. Put them in a doorway to start, (catch yourself on) and you might want to use platform pedals.
Unless you have time to ride during the day- then get the heck outside. Bad weather builds character .

missliz