There is a range in between walking speed 4-5kmh/h and 100m record sprinting 37.58km/h (Usain Bolt 2009). Generally for longer distances average speeds are half those of professionals, so for a marathon 2 hours becomes 4 hours or 10.5km/h. Average speed as you can see really depends on the distance, once you've decided this then Yih can use my rule of thumb of twice pros for a feel. Good luck!