I've exercised the past two days on a bike, and have in the past, but never notice any correlation between that and my writing. I also haven't noticed it improving my attitude. (I keep getting told that exercise is good for depression but so far, and despite efforts in the past, nada. I find that it might distract me while I'm doing it, but when it's over, the bad feelings come right back.)
If there ever is any relation, I think it might be because, while your body is working out, your brain has to find something else to do, so you might think more indepth about writing without being distracted by other things. I know that all sorts of thoughts go through my head when I'm on an exercise bike or taking a walk. Maybe some of those ideas could spark something for a story, but aside from that, working out doesn't seem to energize me to write or anything. But that's just me. *shrug*
*coming from someone who is rather sedentary but loves walking, if there were just more interesting places to walk around here*