Hi there. I also have AS (I'm 35 y/o male), so I feel your pain. I believe the key to success is getting your pain levels down to a manageable state, then you can begin building some muscle and gaining mobility, which will slow the spinal fusing, and also will make the weight loss possible. A couple years ago I was in agonizing pain but have turned things around and am almost normal these days. If you are not a member, you should join kickas.org . it is a great site w/ tons of info and support from members. Many people (including myself) have found relief thru diet or many of the new wonder drugs such as remicade.
Here is an exercise DVD that I bought a while back which is pretty good:
Spondylitis Association of America: Online Store
Any water exercise is great. maybe water aerobics or just swimming laps. I also find the elliptical trainer very useful and it doesn't bother my hips or anything like running does. I was able to get my inflammation down enough to start a weight lifting routine and that has helped me more than anything I believe. But it's all about baby steps and not injuring or aggravating any joints so take it easy. feel free to email me if you I can help. firstname.lastname@example.org