I'm always on the Millennium Coastal Path too. I actually park right where you do, Kev. If I do a half marathon around there (rarely), then I basically run to one side of the path (over the bridge to the car park), and then back to the visitor centre to the end of the footpath and back to the car again. That's pretty much 21K. It's a beautiful run too - lots of variety and a challenge when there's a bit of wind up.
I also love running from around Swansea University into Mumbles and back. Again, if you're feeling adventurous than you can do a nice run from Mumbles around the cliff paths (I warn you, it gets REALLY steep - don't do a recovery run there) to Caswel Bay and beyond.