A spin class and a swim is better than nothing but its not going to get you fit.
it will get you fitter than doing nothing
If I do a spin class I take little notice of the instructor and use it as an interval session
if you get a good instructor this should be part of the class including hill climbs and sprints
Spin classes are a great cardio workout but dont really relate to real world biking too much.
The last spin class l went to i took my HRM, we were going for an hour and when i finished my average was 97% of max…now that cant be good for anybody, and in fact it could lead to burnout (it did with myself) esp if you are cycling regularly too.
your in control of the resistance just turn it down and you won’t burn yourself out.
i’d pop down to your local gym and have a talk to the instructor, explain what you want to achieve and i’m sure they will be able to tell you if spinning is right for you or to try something different