If you aren't heading to Swansea this weekend there is an alternative near Presteigne hosted by our friends in Presteigne Pacers. The annual 2 Rivers Run takes place at 11am with a T shirt as compensation for your efforts. Worth a trip if you are at a loose end , its a nice run and you dont even have to swim any of the rivers.
https://sites.google.com/site/presteignepacersrunning/2-rivers