what could it mean when my 2005 Subaru Baja can no longer go up a mountain road without laboring excessively?

If the accelerator needs to be tapped to return to closed then that is most likely a sticky throttle line or throttle body. This is an easy fix as all that is required is for you to get under the hood and remove the air cleaner hose that goes to your throttle body and remove it. Then, using carb & choke cleaner spray some around the restrictor plate and move it by hand or have someone get in the cab and press the gas pedal, then just wipe it out with a paper towel as terry towels can leave lint and crap stuck in the throttle body.

As far as it running laboured, check for oil leaks around the cylinder head cause you could possibly have a blown head gasket. If it's loud like it has an exhaust leak than it probably does have an exhaust leak since that would also make your engine work harder. Best thing would be to take it in to your local shop and have them do a scan of it.