What is the resistance of a low-pass filter
The effective output resistance of an RC low pass filter is exactly 20 kohm i.e. it is the resistor you chose. If you want it to drive an LED you are not going to get what you require. For instance, the LED and 1k (output load) will significantly affect the cut-off frequency (make it about 20 times higher) and you won't get the current into your LED because of the series 20 kohm.

You might want to consider an op-amp buffer or maybe a BJT buffer between filter and LED+1kohm.