the spin classes helped my legs to realise how hard cycling can be cos I am not a cyclist and aint been out on my bike for ages, and with doing squats etc in the class and high resistance work it strengthened my legs quite well, I did the classes twice a week and really pushed myself.
I would recommend them if you want to do them alongside your other training ( eg whilst you are sorting a bike out). be prepared for the class to just be full of people who wanna chat tho and gossip whilst doing it - annoyed the pants off me to be honest - but I just got my head down and got on with it cos I knew I had a goal- the tri!
I think the classes deffo give you a good base.
Pain is just weakness leaving the body. http://www.hdcc.co.uk