I started cycling again 2 years ago (after a 10 year hiatus). I was working out at a gym (Bally's) 3x week (M/W/F) and didn't want to disrupt my schedule with a spin class.
This JAN I took the plunge and started the class M/W/F, moving my workouts to T/Th. The first few weeks of classes were really tough, but by the end of the first month I was keeping up with everything the instructor could throw at us.
The instructor was an avid cyclist, so she instructed the class like she 'trained' - none of those 'jumps' or 'power surges' that could be harmful to your knees.
In MAR my family joined the YMCA, so I started going to the one closest to our home. Those 'trainers' might have owned a bike, but the class was not very fun, it wasn't very challenging, nor were the instructors personable. The music was on, but just to listen to - the BPM of the music wasn't matched to the cadence we were supposed to keep.
Since our membership allowed me to go to other Y's, I started going to one closer to work. WOW! The instructor was really prepped - each hour-long routine that he crafted was matched by the right music (BPM = cadence). He was personable, and an avid cyclist. Yes, he incorporated 'jumps' into his routine (and I don't do jumps) so I just crank up the resistance and imagine I have a 60-tooth ring (and keep up with the cadence). I usually end up doing 20+ miles (per the computer) in that hour-long class!
Find yourself a good instructor, do the parts of the routine that you want, and have a lot of fun!
BTW - my pre-spin-class average speed on the open road was ~14MPH. Now it's 17+MPH after 5 months...