Divert to any enroute alternate where i can still be within the requirements. If that is not possible i have to declare a PAN PAN for low fuel and act accordingly.
Never had that except in the simulator so far, quite the contrary we usually land with around 500 to 1000gk above planned remaining due to shortcuts enroute. We use for planning longest departure and arrival including any relevant transition procedures so our flown route is usually shorter than the planned one, domestic by up to 100NM.