I've checked the throttle cables and returns springs - the throttle assembly itself appears to be in good working order. I can manually rotate the throttle to full and it returns to the normal position, but when I start it, it's like the accelerator is mashed to floor.

Any suggestions on where to look next? This is a non-turbo 850.

 

I would be suspicious of a vacuum leak

 

Second the vacuum leak

 

When did it start, had you done any work to it recently ??

When it's off is the throttle sitting on the throttle stop screw ??

You might want to try to clean the Idle Air Control motor in case the shutter is seized.
If you pull he IAC motor and plug the two hoses and try to start it and it doesn't run way up then you know it's an IAC problem.
Be careful you use something big enough it doesn't accidentally get sucked in through the IAC hoses when starting
It's just a guess, something to try.