A few years ago I had been in my best shape (daily exercise, cardio and weight lifting) and started experiencing some forearm pain. I thought it was just from over use from typing but i finally went to a physical therapist and found that the pain came from me hunching over on my computer and putting a lot of strain on my fore arms. The posture I took on was a combination of
1. Just leaning in to my computer because I'd be trying to read text or getting "into" whatever i'm working on
2. My upper back not having the endurance or development that I needed to hold up my upper body.
I had to change my exercise routine to give my muscles more endurance to hold my upper back up while seated.
So yea, exercise is definitely going to help but I wouldn't just say it fixes things automagically for you and you could run into posture issues even when you are doing the right things in life. :)