Think the half follows the river for a good section, so a lovely route tho quite different to road. I run on grass a lot in the summer tho along rivers close to me. Barcelona is gorgeous, been there a few times, tho never to run.
Jenf, good luck! I'd love to do a mara like snowdon the one I really want to try is the beachyhead marathon, looks amazing although I'd have to walk good sections of it!