You have kind of answered your own question in that a new activity will strain all sorts of muscles you have previously taken for granted, and you're really going for it....but this is a reoccuring specific problem, not just all round "phew that was hard work, cor my body aches now' type of stuff. I started surfing not much earlier than you, Im very unfit and generally neglect my health, I spent less time surfing in the last year than you probably have in the last month - I still wouldnt expect that acute kind of problem described if I had been surfing as much.
You probably need to see a specialist of some sort (chiropractor/oesteopath - I prefer the latter) who can get to the root cause of the problem and commence treatment. When they do, they will be able to help you out with exercises and address your posture issues. Posture is usually the cause of the issue, mine is terrible (she says slouching in the chair, cross legged (with a hip ache)) so perhaps try some yoga and pilates and concentrate on using your core for balance - not your back and side muscles.
Apologies if I havent really addressed your problem and just rambled, I do that a lot. Especially when Ive been to the pub after work, like tonight
Found a good free yoga class site - you do have to register to access but its free and I like it, so you may like to take a look:http://www.doyogawithme.com