I have the exact same problem. Activity will flare me.
However, over the past two years I have gone from not being able to carry one small grocery bag, or apply enough pressure with a screwdriver to put a screw in a drywall, to getting back into hiking, mountain biking (1-2hrs), and this winter, even a little skate skiing, and that's a workout...
To do that amount of high impact activity though, I have had to keep the diet incredibly 'clean'. NSD is not enough to keep overall levels of inflammation down for me, I follow the autoimmune protocol instead, and have to resort to 400mg ibuprofen 3x a day for the next two days, especially after a skate ski session.
I believe one's overall inflammation-level needs to be brought down, way down, by steady & on going (=permanent) starch/sugar control, in order for the body to be able to adjust to increased activity levels. And this happens over a longer 'recovery timeframe', in my opinion.
Everyone's case is different. My response to diet has been long, clearly NSD is not enough for me, whereas it is for so many others.
Because I'm determined to try and continue to up my amount of sports/activities, I've also managed to wear down my lovely Rheumatologist, who kindly at least listens, and who has 'come around' to rx LDN, which I'm now taking intial doses of as well. (Thanks to 'Sue22' for your posts on LDN & enthesitis.)
I think for those who struggle with activity flaring you without fail, starch/sugar control is even more important. There's no doubt that I will never be 'normal'. I couldn't run a marathon, or ride for as long as non-spondy people, but I have seen an incredible improvement in what I can do physically, and I know it will be an on going maintenace piece for me, to continue to be active, and to slowly, slowly continue to improve.
Time, patience, reduced sugar in all its evil forms = better
