I've been doin spin for about 4 years....started at 50. It's a heck of a workout. I agree with all of the above comments regarding having the bike set up properly and having the resistance adjusted properly. A good instructor will notice poor set-ups and suggest improvements. I too have a torn meniscus and find that spinning doesn't bother it because there is very little lateral movement required. As for not having a cadence meter, speedometer, or odometer....I have a pretty good heart rate monitor that also records effort expended. I recommend having one. Afterall, it's really all about aerobic conditioning and heart beat rates. Most of us in the class have HRM's and track our history. Hope this helps.
Gary