Here we consider the puzzle of long-runout turbidity currents. It is well known, through direct evidence of the flows or from the morphology that they create, that turbidity currents can run out over 100 km in lakes and reservoirs, and over 1000 km in the ocean. The currents do so without dissipating themselves via the excess entrainment of ambient water. Existing layer-averaged formulations are, however, unable to capture this behavior. Here we use the formalism of a Turbidity Current with a Roof to show that the flow partitions itself into two layers. The lower driving layer approaches an asymptotic behavior with invariant flow thickness, velocity profile and suspended sediment concentration profile. The upper rarified layer continues to entrain ambient water indefinitely, but the concentration in that layer becomes ever more dilute, and the layer ultimately has little interaction with the driving layer. This partition likely allows the driving layer to run out long distances, and to follow morphology of its own creation such as leveed subaqueous channels.