Hi Steve,
I was very active most of my life and in great shape also, or so I thought. I blamed my soreness that I experienced for so many years on my active lifestyle. AS hit me hard in my early 30's and come to find out I had had it since a teenager. About 5 years after my diagnosis I ended up in a wheelchair, because it was so aggressive.
Granted I still try to walk and do things, but the pain and stiffness has me limited.
According to my doc, my sternum and most of my ribs are fused, my mid back and right now my SI's and hips are in the process. Some of the fusing seemed to go quick, but other areas take their sweet time. When I was a teenager I dealt with costochondiritis (inflammation of the sternum and ribs) on and off and nobody knew what was causing it. Come to find out it was the AS. So it took all that time up until now for the fusing to happen.
I do know I would much rather deal with the pain of the stiffness after the fusing than the pain caused by the inflammation.

Like everyone has said here...everyone is different and some go years of having pain and still don't fuse. Others fuse really quick with only a short amount of time in pain.
Point being...it's hard to say if you're going to fuse or not. Hopefully you don't and you continue to be as fortunate as you have up until now.

Hugs,
Lisa