If O'Brien lost twenty years of his total lifespan, this would not make him concerned until, oh, about twenty years before his statistically probable age of death. Humans are immortal until they die...
Also, the prison experience would not have "weakened" or "hurt" O'Brien as such - it would merely have made him 20 years older, in the middle of his prime where such things barely show to the surface. More significantly, it would only have made his brain older, as his muscles and bones were not involved much in the pseudo-activity that in fact only lasted for a couple of hours or so. Something like that would simply not be observable to the outside, or even to the inside; Bashir might notice something amiss, but O'Brien himself would not.
Timo Saloniemi
But we already know that the simulation didn't affect O'Brien physically at all, the experience was one hundred percent psychological. What I meant was that even the psychological aspect has been taken care of and that O'Brien doesn't show any sign of having been affected long-term-ly by it at all. No new phobias or weaknesses. He even talks about his past with Kira in that Par March thingy and never alludes to the twenty years gap between now and say the beginning of DS9. The episode doesn't have any long term impact on O'Brien's life. I've watched the series many times and if there was anything, even a subtle reference, I would know about it.