14-15 mph is a good average over a considerable distance. One "beginner" versus another will be different but that is where i normally average. I ride 20-30 miles on gravel roads plus some pavement every day.
Wind, tire pressure, tire tread, air temperature, your conditioning, road surface-pavement, dirt, mud etc. will all play a part in the ultimate average. (not nec. in that order)
I am amazed that no matter how far our fast i seem to ride i normally end up in the 14 mph range on my daily ride. This is done at a fairly good clip but not a pace that is overly exerting.