I was pretty confident in my GSD pup, had him off leash in the mountains and he never strayed too far but when I took him hiking on Monday(at an on leash only place) he about ripped my arm off trying to grab a bird so now I'm not so sure what would happen if he saw something.
Possibly try a longer light weight training lead, I've got one for 50ft but it's a bit cumbersome so a 20 or 30 might work better and they're a lot cheaper then Flexi's.
I do take mine to the dog park occasionally, maybe have a look at the ones around you and scope them out to see what kind of stuff goes on. Going to them in the early morning or right before sunset can decrease your chances of running into the irresponsible owners... I know a few times we've been at the dog park at sunset we have the entire park to ourselves, it was great!
~Ollie vom Grunenfeld HIC(8/29/2012)WGSL/WGWL/DDR cross - herding dog in training!