The basic problem is speed differential. It doesn’t matter whether the faster MUP traveler is running or riding a bike; if the speed difference it too great (whatever that might mean) --- there will be conflict.

When I ride on a MUP, I consider everyone I approach from behind to be completely unaware of me and act accordingly (i.e. I go slow and announce my presence). I also act as though some of them are self-involved folks who are at least one standard deviation down on the IQ scale. ;o)

Joe