2002 Subaru outback is leaking antifreeze and not heating up.

Is a visible external leak noticed.....check engine oil dipstick for internal leak (milky oil).....also milky under the engine oil fill cap.....do a compression test for indication of loss of head gasket integrity......change related cooling system components when replacing a head gasket, ie thermostat, water pump, all hoses. viscous fan clutch, radiator cap......

This is common issue early sign head gaskets failure for EJ251 SOHC engine for external coolant leak. Second gen 96-99 with EJ25D is worst because when head gaskets failed, it's an internal leak that allows coolant and oil mix together in the combustion chamber to destroy engine.
You can drive it for at least 6 months or more as long as you keep top up coolant since the leak should be small at a slower rate leak. I drove mine for a while before change head gaskets. You're looking at `$1000-1200 job a dealership since you might as well change the timing belt, water pump, idlers while engine is pulled to replace head gaskets.