Hi there - like any surgery, there will be some pain after WLS; however, you should not suffer from uncontrolled or unbearable pain. Talk to your surgeon before-hand about what the plan will be for pain-control and what the fallback will be if that doesn't work. In my case, I had a lot of post-op pain and my surgeon had to increase my dose of pain meds to get it under control. Runaway pain hinders healing, so don't try to tough it out, work with your doctor to reduce it to a reasonable level. Good luck, whatever you decide!
I started working out about a year after surgery. Not only did I not gain weight (besides a temporary pound or two) but adding muscle mass really boosted my metabolism and helped me keep losing.