I have a Trane XB1000 unit and the fan has been contstantly running even though the heat and ac have not been on and it's about 40 degrees outside. I just replaced the capacitor by the way. Any thoughts on why it is running all the time?

The model number you posted is for the outside unit only. Is the fan running outside OR is the fan/blower running inside?

Outside unit only. It just has been running for the past 24 to 36 hours.

Is the compressor running also? Try this: thermostat set to normal heating mode, fan set to on, set temp lower than room temp. Does the system continue to heat? If so, the outdoor contactor may not be disengaging to allow the unit to stop. Usually pitted/fused contacts cause this. Turning off power supply to outdoor unit will halt operation.