There are buses from the airport island to the mainland just across the bridge, but it's not terribly interesting or pedestrian friendly - and it's not impossible to get to Namba and back in five hours. That's what I'd do - follow the advice on the six hour thread:
http://www.flyertalk.com/forum/japan...kix-osaka.html

Keep an eye on the clock as you're entering the country - If you're not running on schedule, just eat something in the landside restaurants at the airport. They're about as good as what you're likely to find just across the bridge.