A free 5km timed run takes place on Saturday mornings in Crystal Palace park. You will need to register beforehand, and bring along a barcode that you print off once registered. Meet at 9am just inside the Penge entrance. More details are at:
http://www.parkrun.org.uk/CrystalPalace/Home.aspx
I'm not involved in the organisation of this, but have taken part, and found it to be good fun. What better way to start the weekend?