Anxiety Relief from Cardio/Exercise.

Does anyone suffer from extreme episodes of recurring anxiety attacks? I know that exercise (particularly heavy cardio) helps calm my thoughts immediately after, but I am wondering about exercise's long term affects on anxiety and how long it takes to feel it. I have been slacking on my exercise and my anxiety has been intense enough lately to send me back to the gym. How long does it take some of you to notice the long term effects of exercise on fighting anxiety? I am going to keep returning to the gym even after I feel better, I was just wondering what I should expect.
Thanks!! :)