I'm not sure but I suspect it will greatly depend on what you assume about the distribution of possible run lengths. Also it probably depends on how long you observe them for. Obviously if you observer for the whole of your ourward journey and haven't turned when you do turn then they are definitely running further than you.

To make calculations easier lets just talk about the outward journey and say the run finishes when you turn. Lets say you go out on an outward journey of 5km and lets say that run lengths are uniformly distributed between 0 and 10km (so that the mean is 5km, your run length). If you observe someone running for 2 km then you can eliminate those in the 0-2km part of the distribution and so I think you can say the mean run length of that group is 6km. If you see someone for the whole 5km then they are in a group with a mean run length of 7.5km. But the results is highly sensitive to the distribution you assume and where your run length falls in that distribution.