My teenage daughter wants to go to the gym and trainer/spin classes is the only thing she ever likes to do so i encourage it. So I find myself sat outside the class listening to banging tunes it sounds like it could be quite good fun.
So, do i have a go?
I’m 4.3 W/Kg so i am not doing this for weight loss, and i’m not expecting 2x20 min @ sweetspot so what should i expect and how best to approach it?
I go to spin classes every so often at my club. They have Keiser M3 bikes with totally inaccurate power meters but they do connect to my HR monitor, so its kinda interesting to only ride via RPE.
The ‘feel’ of the crank is like wet oatmeal, and the saddle is like a Huffy bike from 1983. Sometimes the music is good. Usually the scenery is good. 
But no matter what, you can get a good cardio workout, despite a fairly unimpressive power-based workout.
Go to a workout class with your daughter!! Way more important to do that stuff once in a while
Other than agreeing with @batwood14, I think you could make it productive by seeing if it could substitute for something like Pettit. My experience with spin classes a thousand years ago is that you can basically cheat. Go as hard or easy as you want even when everybody else is jumping around like crazy. (Hate those pointless “jumps”).
You don’t have power, I’m assuming, so you could use HR and RPE to get a decent 45-60 min endurance spin.
I really wouldn’t try to do their “intervals”.
Can you pick a bike at the back and just do a 2x20? Not like the instructor knows what’s going on I’m thinking. Use your Apple Watch or whatever to use your HR as a guide…
Or is it harder to pull off that it sounds?
X2. Your daughter will be gone before you know it.
If it’s a goofy spinning class with a bunch of stuff that won’t help with cycling you can also just modify the spin class to meet your needs. You’d still be there with your daughter though.
Your teenage daughter will quickly not want to spend any time with at all. So definitely go with her and forget the watts/kgs and enjoy.
You definitely won’t lose fitness from it either.
Way back when, i lived around the corner from a spin studio. I’d get home from work and walk the 4min and start a session. There was a “good” instructor(?) who led the indoor cycling session, often with TdF footage showing and certainly trying to prep people for outdoor riding (I think in part to get new coaching / life-coach clients). But his session was just 50min or so. Not enough for me. So I went to the class before his, which fit perfectly with my schedule after work. The instructor for this class was a “typical” (in my mind) spin instructor doing jumps, forward / back thingies, and telling people to draw energy from their anger toward her ex boyfriend (or something similarly ridiculous…) which I tunes out, focused on my thing (even her music was terrible) to put in a workout before the “real” session. I kept riding through the end of that class and into the next.
Funny thing was her class was packed and his, while well attended, always had several empty bikes. And I got in two hours “in the saddle” after work. (And with an enjoyable visual atmosphere… this was in Brentwood/Santa Monica, Calif. on Montana Ave. My girlfriend at the time, now my wife, would often come join the second session.)
Do your thing with your daughter. You may want to tell the spin instructor you’re doing your own thing so she doesn’t harp on you or get insulted.
Go to class! I teach two spin classes per week at my local Y. Every instructor is different. In my class, we train to ride outside. (Many don’t actually ride outside but they still like the workouts.). We do FTP tests every few months. Last spring we followed an 8-week Carmichael training plan in prep for the Iron Horse ride in CO. If you don’t like the instructor, or it’s a little too “aerobics class” for you, just do your own thing. Chances are the instructor won’t really notice or care.
I’ve had mixed experiences. I did some classes last year with my wife; some instructors were really good, with useful drills, some were quite bad, encouraging very high load intervals at very low cadence (and I was just turning the pedals at 70-75rpm in the back, ignoring her). I also found that the absence of a freewheel made my pedaling lazy, and it took me a few days back on the trainer to make high-cadence pedaling smooth again without knocking. But this last point is no different than riding on a fixie.