I find that the more fit I get for riding bikes (road bike or bike on trainer or spin bike), the longer it takes me to warm up. Often, I need a good 45 minutes. In a spin class, I may start feeling better closer to 30 min in because of all the high heart rate intervals they tend to make you do early in the class. However, if your spin class is only 50-60min long, then that sucks for improving your fitness really. I don't have access to this type of schedule now, but I used to like to do back to back hour classes plus my own warm up and cool down. It's a little easier to do that than to spend 2+ hours on an indoor trainer, but I'd still make the workout my own in the spin class. I'd advocate trying to find at least 75min classes or take back to back classes occasionally first before just adding more short classes into your week as far as training better. Balance longer more endurance-type workouts with the shorter classes that focus on intervals. This may require some experimenting with different instructors. If you can't do back to back classes but have access to a spin bike outside of class, maybe do an hour warmup on your own, take the class, and then extend your cooldown somehow, whether you just spin through the stretch portion and then stretch on your own or you finish the class as usual and then do an easy spin for a few minutes. Ultimately, no matter which you do, so long as you aren't experiencing symptoms of overtraining, you are not doing anything bad for yourself, and you can only be doing good in burning more calories and improving your fitness.