It depends on how you define the average distance, expected distance in the next period of time or estimated distance of a certain point within the coming period?
Its very difficult to measure the average distance particular when both the nodes are mobile node. However, it can be done in traditional manner. In computer network, using hello and echo packet, it can be done. Available BW (bits/sec) is known, packet size (bits) is also known. Masure round trip time t(sec.) using counter. d = BW * t
To get an accuracy get approximately 10 or 20 readings and do average of it.